SQL Interview
What are the Different Types of Triggers?
1) DML Trigger There are two types of DML Triggers 1.Instead of Trigger Instead of Triggers are fired in place of the triggering action such as an insert, update, or delete. 2. After Trigger After triggers execute following the triggering action, such as an insert, update, or delete. 2) DDL Trigger This type of trigger is fired against Drop Table, Create Table, Alter Table or Login events. DDL Triggers are always After Triggers.
What is a Fact Table?
Fact table contains measurements of business process. Fact table contains the foreign keys for the dimension tables. For instance, if your business process is 'paper production', 'average production of paper by one machine' or 'weekly production of paper' will be considered as the measurement of business process.
What Snow Flake Schema?
In Snowflake Schema, each dimension has a primary dimension table, to which one or more additional dimensions can join. The primary dimension table is the only table that can join to the fact table.
What is Difference between GETDATE and SYSDATETIME in SQL Server 2008?
In case of GETDATE, the precision is till milliseconds, and in case of SYSDATETIME, the precision is till nanoseconds.
What is an SQL Server Agent?
The SQL Server agent plays an important role in the day-to-day tasks of a database administrator (DBA). It is often overlooked as one of the main tools for SQL Server management. Its purpose is to ease the implementation of tasks for the DBA, with its full-function scheduling engine, which allows you to schedule your own jobs and scripts.
What does TOP Operator Do?
The TOP operator is used to specify the number of rows to be returned by a query. The TOP operator has new addition in SQL SERVER 2008 that it accepts variables as well as literal values and can be used with INSERT, UPDATE, and DELETE statements.
What is Use of DBCC Commands?
The Transact-SQL programming language provides DBCC statements that act as Database Console Commands for SQL Server. DBCC commands are used to perform the following tasks. Maintenance tasks on database, index, or filegroup. Tasks that gather and display various types of information. Validation operations on a database, table, index, catalog, filegroup, or allocation of database pages. Miscellaneous tasks such as enabling trace flags or removing a DLL from memory.
Where are SQL server Usernames and Passwords Stored in the SQL server?
They get stored in System Catalog Views, sys.server_principals and sys.sql_logins. However, you will not find password stored in plain text.
What is SQLCMD?
sqlcmd is enhanced version of the isql and osql, and it provides way more functionality than other two options. In other words, sqlcmd is better replacement of isql (which will be deprecated eventually) and osql (not included in SQL Server 2005 RTM). sqlcmd can work in two modes - i) BATCH and ii) interactive modes.
What is T-SQL Script to Take Database Offline - Take Database Online?
-- Take the Database Offline ALTER DATABASE [myDB] SET OFFLINE WITH ROLLBACK IMMEDIATE GO -- Take the Database Online ALTER DATABASE [myDB] SET ONLINE GO
How to Enable/Disable Indexes?
--Disable Index ALTER INDEX [IndexName] ON TableName DISABLE GO --Enable Index ALTER INDEX [IndexName] ON TableName REBUILD GO
What is the Maximum Number of Index per Table?
1 Clustered Index + 249 Nonclustered Index = 250 Index. For SQL Server 2008: 1 Clustered Index + 999 Nonclustered Index = 1000 Index.
What are the Different Normalization Forms?
1NF: Eliminate Repeating Groups Make a separate table for each set of related attributes, and give each table a primary key. Each field contains at most one value from its attribute domain. 2NF: Eliminate Redundant Data If an attribute depends on only part of a multi-valued key, then remove it to a separate table. 3NF: Eliminate Columns Not Dependent On Key If attributes do not contribute to a description of the key, then remove them to a separate table. All attributes must be directly dependent on the primary key. (Read more here) BCNF: Boyce-Codd Normal Form If there are non-trivial dependencies between candidate key attributes, then separate them out into distinct tables. 4NF: Isolate Independent Multiple Relationships No table may contain two or more 1:n or n:m relationships that are not directly related. 5NF: Isolate Semantically Related Multiple Relationships There may be practical constrains on information that justify separating logically related many-to-many relationships. ONF: Optimal Normal Form A model limited to only simple (elemental) facts, as expressed in Object Role Model notation. DKNF: Domain-Key Normal Form A model free from all modification anomalies is said to be in DKNF. Remember, these normalization guidelines are cumulative. For a database to be in 3NF, it must first fulfill all the criteria of a 2NF and 1NF database.
What is the 'FILLFACTOR'?
A "FILLFACTOR" is one of the important arguments that can be used while creating an index. According to MSDN, FILLFACTOR specifies a percentage that indicates how much the Database Engine should fill each index page during index creation or rebuild. Fill-factor is always an integer valued from 1 to 100. The fill-factor option is designed for improving index performance and data storage. By setting the fill-factor value, you specify the percentage of space on each page to be filled with data, reserving free space on each page for future table growth. Specifying a fill-factor value of 70 would imply that 30 percent of each page will be left empty, providing space for index expansion as data is added to the underlying table. The fill-factor setting applies only when the index is created or rebuilt.
What is CHECK Constraint?
A CHECK constraint is used to limit the values that can be placed in a column. The check constraints are used to enforce domain integrity.
What is FOREIGN KEY?
A FOREIGN KEY constraint prevents any actions that would destroy links between tables with the corresponding data values. A foreign key in one table points to a primary key in another table. Foreign keys prevent actions that would leave rows with foreign key values when there are no primary keys with that value. The foreign key constraints are used to enforce referential integrity.
What is NOT NULL Constraint?
A NOT NULL constraint enforces that the column will not accept null values. The not null constraints are used to enforce domain integrity, as the check constraints.
What is PRIMARY KEY?
A PRIMARY KEY constraint is a unique identifier for a row within a database table. Every table should have a primary key constraint to uniquely identify each row, and only one primary key constraint can be created for each table. The primary key constraints are used to enforce entity integrity.
What is PIVOT and UNPIVOT?
A Pivot Table can automatically sort, count, and total the data stored in one table or spreadsheet and create a second table displaying the summarized data. The PIVOT operator turns the values of a specified column into column names, effectively rotating a table. In simpler word UNPIVOT table is reverse of PIVOT Table, however it is not exactly true. UNPIVOTING is for sure reverse operation to PIVOTING but if during PIVOTING process data aggregated the UNPIVOT table does not return to original table.
What is Use of @@ SPID in SQL Server?
A SPID is the returns sessions ID of the current user process. And using that session ID, we can find out that the last query was executed.
What is UNIQUE KEY Constraint?
A UNIQUE constraint enforces the uniqueness of the values in a set of columns; so no duplicate values are entered. The unique key constraints are used to enforce entity integrity as the primary key constraints.
What are the Difference between Clustered and a Non-clustered Index?
A clustered index is a special type of index that reorders the way records in the table are physically stored. Therefore, the table can have only one clustered index. The leaf nodes of a clustered index contain the data pages. A non-clustered index is a special type of index in which the logical order of the index does not match the physical stored order of the rows on disk. The leaf node of a non-clustered index does not consist of the data pages. Instead, the leaf nodes contain index rows. (Read more here)
What is a Cursor?
A cursor is a database object used by applications to manipulate data in a set on a row-by-row basis, instead of the typical SQL commands that operate on all the rows in the set at one time. In order to work with a cursor, we need to perform some steps in the following order: Declare cursor Open cursor Fetch row from the cursor Process fetched row Close cursor Deallocate cursor (Read more here)
What is a Data Mart?
A data mart (DM) is a specialized version of a data warehouse (DW). Like data warehouses, data marts contain a snapshot of operational data that helps business people to strategize based on analyses of past trends and experiences. The key difference is that the creation of a data mart is predicated on a specific, predefined need for a certain grouping and configuration of select data. A data mart configuration emphasizes easy access to relevant information (Reference: Wiki). Data Marts are designed to help the manager make strategic decisions about their business.
What is Data Warehousing?
A data warehouse is the main repository of an organization's historical data, its corporate memory. It contains the raw material for management's decision support system. The critical factor leading to the use of a data warehouse is that a data analyst can perform complex queries and analysis, such as data mining, on the information without slowing down the operational systems (Ref: Wikipedia). Data warehousing collection of data designed to support management decision making. Data warehouses contain a wide variety of data that present a coherent picture of business conditions at a single point in time. It is a repository of integrated information, available for queries and analysis.
What is Dirty Read?
A dirty read occurs when two operations, say, read and write occur together giving the incorrect or unedited data. Suppose, A changed a row but did not committed the changes. B reads the uncommitted data but his view of the data may be wrong so that is Dirty Read.
What is a Factless Facts Table?
A fact table that does not contain numeric fact columns is called a factless facts table.
What is the Difference between a Local and a Global Temporary Table?
A local temporary table exists only for the duration of a connection, or if defined inside a compound statement, for the duration of the compound statement. A global temporary table remains in the database accessible across the connections. Once the connection where original global table is declared dropped this becomes unavailable.
What are Lookup Tables?
A lookup table is the table placed on the target table based upon the primary key of the target; it just updates the table by allowing only modified (new or updated) records based on the lookup condition.
How does Using a Separate Hard Drive for Several Database Objects Improves Performance Right Away?
A non-clustered index and tempdb can be created on a separate disk to improve performance.
What is Junk Dimension?
A number of very small dimensions may get lumped together to form a single dimension, i.e. a junk dimension - the attributes are not closely related. Grouping of Random flags and text Attributes in a dimension and moving them to a separate sub dimension is known as junk dimension.
What is a View?
A simple view can be thought of as a subset of a table. It can be used for retrieving data as well as updating or deleting rows. Rows updated or deleted in the view are updated or deleted in the table the view was created with. It should also be noted that as data in the original table changes, so does the data in the view as views are the way to look at parts of the original table. The results of using a view are not permanently stored in the database. The data accessed through a view is actually constructed using standard T-SQL select command and can come from one to many different base tables or even other views.
What are Sparse Columns?
A sparse column is another tool used to reduce the amount of physical storage used in a database. They are the ordinary columns that have an optimized storage for null values. Sparse columns reduce the space requirements for null values at the cost of more overhead to retrieve non-null values.
What is a Stored Procedure?
A stored procedure is a named group of SQL statements that have been previously created and stored in the server database. Stored procedures accept input parameters so that a single procedure can be used over the network by several clients using different input data. And when the procedure is modified, all clients automatically get the new version. Stored procedures reduce network traffic and improve performance. Stored procedures can be used to help ensure the integrity of the database. e.g. sp_helpdb, sp_renamedb, sp_depends etc.
What is a Surrogate Key?
A surrogate key is a substitution for the natural primary key. It is just a unique identifier or number for each row that can be used for the primary key to the table. The only requirement for a surrogate primary key is that it should be unique for each row in the table. It is useful because the natural primary key can change and this makes updates more difficult. Surrogated keys are always integer or numeric.
What are the Different Index Configurations a Table can have?
A table can have one of the following index configurations: No indexes A clustered index A clustered index and many non-clustered indexes A non-clustered index Many non-clustered indexes
What is a Trigger?
A trigger is a SQL procedure that initiates an action when an event (INSERT, DELETE or UPDATE) occurs. Triggers are stored in and managed by the DBMS. Triggers are used to maintain the referential integrity of data by changing the data in a systematic fashion. A trigger cannot be called or executed; DBMS automatically fires the trigger as a result of a data modification to the associated table. Triggers can be considered to be similar to stored procedures in that both consist of procedural logic that is stored at the database level. Stored procedures, however, are not event-drive and are not attached to a specific table as triggers are. Stored procedures are explicitly executed by invoking a CALL to the procedure while triggers are implicitly executed. In addition, triggers can also execute stored procedures. Nested Trigger: A trigger can also contain INSERT, UPDATE and DELETE logic within itself; so when the trigger is fired because of data modification, it can also cause another data modification, thereby firing another trigger. A trigger that contains data modification logic within itself is called a nested trigger. (Read more here)
What is the Difference between a View and a Materialized View?
A view takes the output of a query and makes it appear like a virtual table; and it can be used in place of tables. A materialized view provides indirect access to table data by storing the results of a query in a separate schema object.
How is ACID property related to Database?
ACID (an acronym for Atomicity Consistency Isolation Durability) is a concept that Database Professionals generally look for while evaluating databases and application architectures. For a reliable database, all this four attributes should be achieved. Atomicity is an all-or-none proposition. Consistency guarantees that a transaction never leaves your database in a half-finished state. Isolation keeps transactions separated from each other until they are finished. Durability guarantees that the database will keep track of pending changes in such a way that the server can recover from an abnormal termination.
Explain IntelliSense for Query Editing:
After implementing IntelliSense, we will not have to remember all the syntax or browse online references. IntelliSense offers a few additional features besides just completing the keyword.
What is Aggregate Functions?
Aggregate functions perform a calculation on a set of values and return a single value. Aggregate functions ignore NULL values except COUNT function. HAVING clause is used, along with GROUP BY for filtering query using aggregate values. The following functions are aggregate functions. AVG, MIN, CHECKSUM_AGG, SUM, COUNT, STDEV, COUNT_BIG, STDEVP, GROUPING, VAR, MAX, VARP
What is Active Data Warehousing?
An active data warehouse provides information that enables decision-makers within an organization to manage customer relationships nimbly, efficiently and proactively.
What are Aggregate Tables?
An aggregate table contains the summary of existing warehouse data, which is grouped to certain levels of dimensions. It is always easy to retrieve data from aggregated tables than visiting original table which has millions of records. Aggregate tables reduce the load in the database server and improve the performance of the query, and they also can retrieve the result quickly.
What is an Execution Plan? When would you Use it? How would you View the Execution Plan?
An execution plan is basically a road map that graphically or textually shows the data retrieval methods chosen by the SQL Server query optimizer for a stored procedure or ad-hoc query, and it is a very useful tool for a developer to understand the performance characteristics of a query or stored procedure since the plan is the one that SQL Server will place in its cache and use to execute the stored procedure or query. Within the Query Analyzer, there is an option called "Show Execution Plan" (in the Query drop-down menu). If this option is turned on, it will display query execution plan in a separate window when the query is ran again.
What is an Index?
An index is a physical structure containing pointers to the data. Indices are created in an existing table to locate rows more quickly and efficiently. It is possible to create an index on one or more columns of a table, and each index is given a name. The users cannot see the indexes; they are just used to speed up queries. Effective indexes are one of the best ways to improve performance in a database application. A table scan happens when there is no index available to help a query. In a table scan, the SQL Server examines every row in the table to satisfy the query results. Table scans are sometimes unavoidable, but on large tables, scans have a terrific impact on performance.
What is the Difference between Index Seek vs. Index Scan?
An index scan means that SQL Server reads all the rows in a table, and then returns only those rows that satisfy the search criteria. When an index scan is performed, all the rows in the leaf level of the index are scanned. This essentially means that all the rows of the index are examined instead of the table directly. This is sometimes compared to a table scan, in which all the table data is read directly. However, there is usually little difference between an index scan and a table scan. An index seek, on the other hand, means that the Query Optimizer relies entirely on the index leaf data to locate rows satisfying the query condition. An index seek will be most beneficial in cases where a small percentage of rows will be returned. An index seek will only affect the rows that satisfy a query condition and the pages that contain these qualifying rows; in terms of performance, this is highly beneficial when a table has a very large number of rows.
What is BCP? When is it Used?
BCP or BulkCopy is a tool used to copy huge amounts of data from tables and views. BCP does not copy the complete structures from source to destination. BULK INSERT command helps to import a data file into a database table or view in a user-specified format.
What languages BI uses to achieve the goal?
BI uses following languages for achieve the Goal. MDX - Multidimensional Expressions: This language is used for retrieving data from SSAS cubes. It looks very similar to T-SQL, but it is very different in the areas of conceptualization and implementation. DMX - Data Mining Extensions: This is again used for SSAS, but rather than cubes it is used for data mining structures. This language is more complicated than MDX. Microsoft has provided many wizards in its BI tools, which further reduced number of experts for learning this language, which deals with data mining structures. XMLA - XML for Analysis: This is mainly used for SSAS administrative tasks. It is quite commonly used in administration tasks such as backup or restore database, copy and move database, or for learning Meta data information. Again, MS BI tools provide a lot of wizards for the same. (Read More Here)
What is BUS Schema?
BUS Schema consists of a master suite of confirmed dimension and standardized definition of facts.
What's the Difference between a Primary Key and a Unique Key?
Both primary key and unique key enforce uniqueness of the column on which they are defined. But by default, the primary key creates a clustered index on the column, whereas unique key creates a non-clustered index by default. Another major difference is that primary key doesn't allow NULLs, but unique key allows one NULL only. (Read more here)
What is Business Intelligence (BI)?
Business Intelligence (BI) refers to technologies, applications and practices for the collection, integration, analysis, and presentation of business information and sometimes to the information itself. The purpose of BI is to support better business decision making. Thus, BI is also described as a decision support system (DSS). BI systems provide historical, current, and predictive views of business operations, most often using data that has been gathered into a data warehouse or a data mart and occasionally working from operational data.
What is CHECKPOINT Process in the SQL Server?
CHECKPOINT process writes all dirty pages for the current database to disk. Dirty pages are data pages that have been entered into the buffer cache and modified, but not yet written to disk.
What is the CPU Pressure?
CPU pressure is a state wherein the CPU is fully occupied with currently assigned tasks and there are more tasks in the queue that have not yet started.
How to Create Primary Key with Specific Name while Creating a Table?
CREATE TABLE [dbo].[TestTable]( [ID] [int] IDENTITY(1,1)NOTNULL, [FirstName] [varchar](100)NULL, CONSTRAINT [PK_TestTable] PRIMARYKEYCLUSTERED ([ID] ASC)) GO
What is CTE?
CTE is the abbreviation for Common Table Expression. A CTE is an expression that can be thought of as a temporary result set which is defined within the execution of a single SQL statement. A CTE is similar to a derived table in that it is not stored as an object and lasts only for the duration of the query.
What are Different Types of Collation Sensitivity?
Case sensitivity - A and a, B and b, etc. Accent sensitivity - a and á, o and ó, etc. Kana Sensitivity - When Japanese Kana characters Hiragana and Katakana are treated differently, it is called Kana sensitive. Width sensitivity - When a single-byte character (half-width) and the same character represented as a double-byte character (full-width) are treated differently, it is width sensitive. (Read more here)
What is Catalog Views?
Catalog views return information that is used by the SQL Server Database Engine. Catalog Views are the most general interface to the catalog metadata and provide the most efficient way to obtain, transform, and present customized forms of this information. All user-available catalog metadata is exposed through catalog views.
What are Conformed Dimensions?
Conformed dimensions mean the exact same thing with every possible fact table to which they are joined. They are common to the cubes.
What is Change Data Capture (CDC) in SQL Server 2008?
Change Data Capture (CDC) records INSERTs, UPDATEs, and DELETEs applied to SQL Server tables and makes a record available of what changed, where, and when, in simple relational 'change tables' rather than in an esoteric chopped salad of XML. These change tables contain columns that reflect the column structure of the source table you have chosen to track along with the metadata needed to understand the changes that have been made.
Why can there be only one Clustered Index and not more than one?
Cluster Index physically stores data, or arranges data in one order (depends on which column(s) you have defined Clustered index and in which order). As a fact, we all know that a set of data can be only stored in only one order; that is why only one clustered index is possible.
What is Collation?
Collation refers to a set of rules that determine how data is sorted and compared. Character data is sorted using rules that define the correct character sequence with options for specifying case sensitivity, accent marks, Kana character types, and character width. (Read more here)
What is a Conformed Fact?
Conformed dimensions are the dimensions which can be used across multiple Data Marts in combination with multiple facts tables accordingly.
What are Different Types of Join?
Cross Join A cross join that does not have a WHERE clause produces the Cartesian product of the tables involved in the join. The size of a Cartesian product result set is the number of rows in the first table multiplied by the number of rows in the second table. The common example is when company wants to combine each product with a pricing table to analyze each product at each price. Inner Join A join that displays only the rows that have a match in both joined tables is known as inner Join. This is the default type of join in the Query and View Designer. Outer Join A join that includes rows even if they do not have related rows in the joined table is an Outer Join. You can create three different outer join to specify the unmatched rows to be included: Left Outer Join: In Left Outer Join, all the rows in the first-named table, i.e. "left" table, which appears leftmost in the JOIN clause, are included. Unmatched rows in the right table do not appear. Right Outer Join: In Right Outer Join, all the rows in the second-named table, i.e. "right" table, which appears rightmost in the JOIN clause are included. Unmatched rows in the left table are not included. Full Outer Join: In Full Outer Join, all the rows in all joined tables are included, whether they are matched or not. Self Join This is a particular case when one table joins to itself with one or two aliases to avoid confusion. A self join can be of any type, as long as the joined tables are the same. A self join is rather unique in that it involves a relationship with only one table. The common example is when company has a hierarchal reporting structure whereby one member of staff reports to another. Self Join can be Outer Join or Inner Join. (Read more here)
What is a Cube and Linked Cube with Reference to Data Warehouse?
Cubes are logical representation of multidimensional data. The edge of the cube contains dimension members and the body of the cube contains data values. The linking in cube ensures that the data in the cubes remain consistent.
What is Data Mining?
Data Mining is the process of analyzing data from different perspectives and summarizing it into useful information.
What is the Difference between OLTP and OLAP?
Data Source OLTP: Operational data is from original data source of the data OLAP: Consolidation data is from various sources. Process Goal OLTP: Snapshot of business processes which do fundamental business tasks OLAP: Multi-dimensional views of business activities of planning and decision making Queries and Process Scripts OLTP: Simple quick running queries ran by users. OLAP: Complex long running queries by system to update the aggregated data. Database Design OLTP: Normalized small database. Speed will be not an issue because of a small database, and normalization will not degrade performance. This adopts the entity relationship (ER) model and an application-oriented database design. OLAP: De-normalized large database. Speed is an issue because of a large database and de-normalizing will improve performance as there will be less tables to scan while performing tasks. This adopts star, snowflake or fact constellation mode of subject-oriented database design. Back up and System Administration OLTP: Regular Database backup and system administration can do the job. OLAP: Reloading the OLTP data is considered as a good backup option. What are Normalization Forms? There are different types of normalization forms such as 1NF: Eliminate Repeating Groups Make a separate table for each set of related attributes, and give each table a primary key. Each field contains at most one value from its attribute domain. 2NF: Eliminate Redundant Data If an attribute depends on only part of a multi-valued key, remove it to a separate table. 3NF: Eliminate Columns Not Dependent On Key If attributes do not contribute to a description of the key, then remove them to a separate table. All attributes must be directly dependent on the primary key BCNF: Boyce-Codd Normal Form If there are non-trivial dependencies between candidate key attributes, then separate them out into distinct tables. 4NF: Isolate Independent Multiple Relationships No table may contain two or more 1:n or n:m relationships that are not directly related. 5NF: Isolate Semantically Related Multiple Relationships There may be practical constrains on information that justify separating logically related many-to-many relationships. ONF: Optimal Normal Form A model limited to only simple (elemental) facts, as expressed in Object Role Model notation. DKNF: Domain-Key Normal Form A model free from all modification anomalies. Remember, these normalization guidelines are cumulative. For a database to be in 3NF, it must first fulfill all the criteria of a 2NF and 1NF database.
What is De-normalization?
De-normalization is the process of attempting to optimize the performance of a database by adding redundant data. It is sometimes necessary because current DBMSs implement the relational model poorly. A true relational DBMS would allow for a fully normalized database at the logical level, while providing physical storage of data that is tuned for high performance. De-normalization is a technique to move from higher to lower normal forms of database modeling in order to speed up database access.
What is the Difference between OLAP and Data Warehouse?
Data Warehouse is the place where the data is stored for analysis, whereas OLAP is the process of analyzing the data, managing aggregations, partitioning information into cubes for in depth visualization.
Why is Data Modeling Important?
Data modeling is probably the most labor intensive and time consuming part of the development process. The goal of the data model is to make sure that the all data objects required by the database are completely and accurately represented. Because the data model uses easily understood notations and natural language, it can be reviewed and verified as correct by the end users. In computer science, data modeling is the process of creating a data model by applying a data model theory to create a data model instance. A data model theory is a formal data model description. In data modeling, we are structuring and organizing data. These data structures are then typically implemented in a database management system. In addition to defining and organizing the data, data modeling will impose (implicitly or explicitly) constraints or limitations on the data placed within the structure. Managing large quantities of structured and unstructured data is a primary function of information systems. Data models describe structured data for storage in data management systems such as relational databases. They typically do not describe unstructured data, such as word processing documents, email messages, pictures, digital audio, and video.
What is Real-Time Data-Warehousing?
Data warehousing captures business activity data. Real-time data warehousing captures business activity data as it occurs. As soon as the business activity is complete and there is data about it, the completed activity data flows into the data warehouse and becomes available instantly.
What is the Difference between Data Warehousing and Business Intelligence?
Data warehousing deals with all aspects of managing the development, implementation and operation of a data warehouse or data mart, including meta data management, data acquisition, data cleansing, data transformation, storage management, data distribution, data archiving, operational reporting, analytical reporting, security management and backup/recovery planning. Business intelligence, on the other hand, is a set of software tools that enable an organization to analyze measurable aspects of their business such as sales performance, profitability, operational efficiency, effectiveness of marketing campaigns, market penetration among certain customer groups, cost trends, anomalies and exceptions. Typically, the term 'business intelligence' is used to encompass OLAP, data visualization, data mining and query/reporting tools. (Reference: Les Barbusinski)
What is Database Mirroring?
Database mirroring can be used with replication to provide availability for the publication database. Database mirroring involves two copies of a single database that typically reside on different computers. At any given time, only one copy of the database is currently available to clients, which is known as the principal database. Updates made by the clients to the principal database are applied to the other copy of the database, known as the mirror database. Mirroring involves applying the transaction log from every insertion, update, or deletion made on the principal database onto the mirror database.
What is Normalization?
Database normalization is a data design and organization process applied to data structures based on rules that help building relational databases. In relational database design, the process of organizing data to minimize redundancy is called normalization. Normalization usually involves dividing a database into two or more tables and defining relationships between the tables. The objective is to isolate data so that additions, deletions, and modifications of a field can be made in just one table and then propagated through the rest of the database via the defined relationships.
What is Use of EXCEPT Clause?
EXCEPT clause is similar to MINUS operation in Oracle. The EXCEPT query and MINUS query return all rows in the first query that are not returned in the second query. Each SQL statement within the EXCEPT query and MINUS query must have the same number of fields in the result sets with similar data types.
What is Difference between DELETE and TRUNCATE Commands?
Delete command removes the rows from a table on the basis of the condition that we provide with a WHERE clause. Truncate will actually remove all the rows from a table, and there will be no data in the table after we run the truncate command. TRUNCATE TRUNCATE is faster and uses fewer system and transaction log resources than DELETE. (Read all the points below) TRUNCATE removes the data by deallocating the data pages used to store the table's data, and only the page deallocations are recorded in the transaction log. TRUNCATE removes all the rows from a table, but the table structure, its columns, constraints, indexes and so on remains. The counter used by an identity for new rows is reset to the seed for the column. You cannot use TRUNCATE TABLE on a table referenced by a FOREIGN KEY constraint. Using T-SQL - TRUNCATE cannot be rolled back unless it is used in TRANSACTION. OR TRUNCATE can be rolled back when used with BEGIN ... END TRANSACTION using T-SQL. TRUNCATE is a DDL Command. TRUNCATE resets the identity of the table. DELETE DELETE removes rows one at a time and records an entry in the transaction log for each deleted row. DELETE does not reset Identity property of the table. DELETE can be used with or without a WHERE clause DELETE activates Triggers if defined on table. DELETE can be rolled back. DELETE is DML Command. DELETE does not reset the identity of the table. (Read more here)
What is Dimensional Modeling?
Dimensional data model concept involves two types of tables and it is different from the 3rd normal form. This concept uses Facts table, which contains the measurements of the business, and Dimension table, which contains the context (dimension of calculation) of the measurements.
What is a Dimension Table?
Dimensional table contains textual attributes of measurements stored in the facts tables. Dimensional table is a collection of hierarchies, categories and logic which can be used for user to traverse in hierarchical nodes.
What is Difference between ER Modeling and Dimensional Modeling?
ER modeling is used for normalizing the OLTP database design. Dimensional modeling is used for de-normalizing the ROLAP/MOLAP design.
What is ETL?
ETL is abbreviation of extract, transform, and load. ETL is software that enables businesses to consolidate their disparate data while moving it from place to place, and it doesn't really matter that that data is in different forms or formats. The data can come from any source. ETL is powerful enough to handle such data disparities. First, the extract function reads data from a specified source database and extracts a desired subset of data. Next, the transform function works with the acquired data - using rules or lookup tables, or creating combinations with other data - to convert it to the desired state. Finally, the load function is used to write the resulting data to a target database.
What is ER Diagram?
Entity Relationship (ER) Diagrams are a major data modeling tool and will help organize the data in your project into entities and define the relationships between the entities. This process has enabled the analyst to produce a good database structure so that the data can be stored and retrieved in a most efficient manner. An entity-relationship (ER) diagram is a specialized graphic that illustrates the interrelationships between entities in a database. A type of diagram used in data modeling for relational data bases. These diagrams show the structure of each table and the links between tables.
What is a Filestream?
Filestream allows you to store large objects in the file system and have these files integrated within the database. It enables SQL Server-based applications to store unstructured data such as documents, images, audios and videos in the file system. FILESTREAM basically integrates the SQL Server Database Engine with New Technology File System (NTFS); it basically stores the data in varbinary (max) data type. Using this data type, the unstructured data is stored in the NTFS file system, and the SQL Server Database Engine manages the link between the Filestream column and the actual file located in the NTFS. Using Transact-SQL statements users can insert, update, delete and select the data stored in FILESTREAM-enabled tables.
Where in MS SQL Server is '100' equal to '0'?
Fill-factor settings of 0 and 100 are equal!
What is Filtered Index?
Filtered Index is used to index a portion of rows in a table that means it applies filter on INDEX which improves query performance, reduces index maintenance costs, and reduces index storage costs when compared with full-table indexes. When we see an Index created with a WHERE clause, then that is actually a FILTERED INDEX.
Describes the Foreign Key Columns in Fact Table and Dimension Table?
Foreign keys of dimension tables are primary keys of entity tables. Foreign keys of facts tables are primary keys of Dimension tables.
What is a Hint?
Hints are options and strong suggestions specified for enforcement by the SQL Server query processor on DML statements. The hints override any execution plan the query optimizer might select for a query.(Read more here) There are three different types of hints. Let us understand the basics of each of them separately. Join Hint This hint is used when more than one table is used in a query. Two or more tables can be joined using different types of joins. This hint forces the type of join algorithm that is used. Joins can be used in SELECT, UPDATE and DELETE statements. Query Hint This hint is used when certain kind of logic has to be applied to a whole query. Any hint used in the query is applied to the complete query as opposed to a part of it. There is no way to specify that only a certain part of a query should be used with the hint. After any query, the OPTION clause is specified to apply the logic to this query. A query always has any of the following statements: SELECT, UPDATE, DELETE, INSERT or MERGE (SQL 2K8); and this hint can be applied to all of them. Table Hint This hint is used when certain kind of locking mechanism of tables has to be controlled. SQL Server query optimizer always puts the appropriate kind of lock on tables, when any of the Transact SQL operations SELECT, UPDATE, DELETE, INSERT or MERGE is used. There are certain cases when the developer knows when and where to override the default behavior of the locking algorithm, and these hints are useful in those scenarios. (Read more here)
What is Hybrid Slowly Changing Dimension?
Hybrid SCDs are combination of both SCD 1 and SCD 2. It may happen that in a table, some columns are important and we need to track changes for them, i.e. capture the historical data for them, whereas in some columns even if the data changes, we do not care.
What is Difference in Performance between INSERT TOP (N) INTO Table and Using Top with INSERT?
INSERT TOP (N) INTO Table is faster than Using Top with INSERT but when we use INSERT TOP (N) INTO Table, the ORDER BY clause is totally ignored.
What is an Identity?
Identity (or AutoNumber) is a column that automatically generates numeric values. A start and increment value can be set, but most DBAs leave these at 1. A GUID column also generates unique keys. Updated based on the comment of Aaron Bertrand. (Blog)
How to get @@ERROR and @@ROWCOUNT at the Same Time?
If @@Rowcount is checked after Error checking statement, then it will have 0 as the value of @@Recordcount as it would have been reset. And if @@Recordcount is checked before the error-checking statement, then @@Error would get reset. To get @@error and @@rowcount at the same time, include both in same statement and store them in a local variable. SELECT @RC = @@ROWCOUNT, @ER = @@ERROR
What is Degenerate Dimension Table?
If a table contains values, which are neither dimension nor measures, then it is called a degenerate dimension table.
What are Points to Remember while Using the FILLFACTOR Argument?
If fill-factor is set to 100 or 0, the Database Engine fills pages to their capacity while creating indexes. The server-wide default FILLFACTOR is set to 0. To modify the server-wide default value, use the sp_configure system stored procedure. To view the fill-factor value of one or more indexes, use sys.indexes. To modify or set the fill-factor value for individual indexes, use CREATE INDEX or ALTER INDEX statements. Creating a clustered index with a FILLFACTOR < 100 may significantly increase the amount of space the data occupies because the Database Engine physically reallocates the data while building the clustered index.
How can I Get Data from a Database on Another Server?
If you want to import data only through T-SQL query, then use OPENDATASOURCE function. To repeatedly get data from another server, create a linked server and then use the OPENQUERY function or use 4-part naming. If you are not adhered with T-SQL, then it is better to use import/export wizard, and you can save it as a SSIS package for future use.
How to Stop Log File Growing too Big?
If your Transaction Log file was growing too big and you wanted to manage its size, then instead of truncating transaction log file, you should choose one of the options mentioned below. 1) Convert the Recovery Model to Simple Recovery If you change your recovery model to Simple Recovery Model, then you will not encounter the extraordinary growth of your log file. However, please note if you have one long running transaction it will for sure grow your log file till the transaction is complete. 2) Start Taking Transaction Log Backup In this Full Recovery Model, your transaction log will grow until you take a backup of it. You need to take the T-Log Backup at a regular interval. This way, your log would not grow beyond some limits.
What is OLTP (Online Transaction Processing)?
In OLTP -(online transaction processing) systems, relational database design uses the discipline of data modeling and generally follows the Codd rules of data normalization in order to ensure absolute data integrity. Using these rules, complex information is broken down into its most simple structures (a table) where all of the individual atomic level elements relate to each other and satisfy the normalization rules.
Explain Object Explorer Enhancements:
In Object Explorer Detail, the new feature is Object Search. Enter any object name in the object search box and the searched result will be displayed in the same window as Object Explorer Detail. Additionally, there are new wizards which help you perform several tasks, from policy management to disk monitoring. One cool thing is that everything displayed in the object explorer details screen can be right away copied and pasted to Excel without any formatting issue.
What is Data Compression?
In SQL SERVE 2008, Data Compression comes in two flavors: Row Compression Page Compression Row Compression Row compression changes the format of physical storage of data. It minimize the metadata (column information, length, offsets etc) associated with each record. Numeric data types and fixed-length strings are stored in variable-length storage format, just like Varchar. (Read more here) Page Compression Page compression allows common data to be shared between rows for a given page. It uses the following techniques to compress data: Row compression. Prefix Compression. For every column in a page, duplicate prefixes are identified. These prefixes are saved in compression information headers which resides after the page header. A reference number is assigned to these prefixes and that reference number is replaced where ever those prefixes are being used. Dictionary Compression Dictionary compression searches for duplicate values throughout the page and stores them in CI. The main difference between prefix and dictionary compression is that the former is only restricted to one column while the latter is applicable to the complete page.
How can I Track the Changes or Identify the Latest Insert-Update-Delete from a Table?
In SQL Server 2005 and earlier versions, there is no inbuilt functionality to know which row was recently changed and what the changes were. However, in SQL Server 2008, a new feature known as Change Data Capture (CDC) has been introduced to capture the changed data.
What is CLR?
In SQL Server 2008, SQL Server objects such as user-defined functions can be created using such CLR languages. This CLR language support extends not only to user-defined functions, but also to stored procedures and triggers. You can develop such CLR add-ons to SQL Server using Visual Studio 2008.
What is the Difference between GRANT and WITH GRANT while Giving Permissions to the User?
In case of only GRANT, the username cannot grant the same permission to other users. On the other hand, with the option WITH GRANT, the username will be able to give the permission after receiving requests from other users.
How do We Know if Any Query is Retrieving a Large Amount of Data or very little data?
In one way, it is quite easy to figure this out by just looking at the result set; however, this method cannot be relied upon every time as it is difficult to reach a conclusion when there are many columns and many rows. It is easy to measure how much data is retrieved from server to client side. The SQL Server Management Studio has feature that can measure client statistics.
What is the Difference between VARCHAR and NVARCHAR datatypes?
In principle, they are the same and are handled in the same way by your application. The only difference is that NVARCHAR can handle unicode characters, allowing you to use multiple languages in the database (Arabian, Chinese, etc.). NVARCHAR takes twice as much space when compared to VARCHAR. Use NVARCHAR only if you are using foreign languages.
Why the Trigger Fires Multiple Times in Single Login?
It happens because multiple SQL Server services are running and also as intellisense is turned on.
What is a Covered index?
It is an index that can satisfy a query just by its index keys without having needed to touch the data pages. It means that when a query is fired, SQL Server doesn't need to go to the table to retrieve the rows, but can produce the results directly from the index as the index covers all the columns used in query.
What is LINQ?
Language Integrated Query (LINQ) adds the ability to query objects using .NET languages. The LINQ to SQL object/relational mapping (O/RM) framework provides the following basic features: Tools to create classes (usually called entities) mapped to database tables Compatibility with LINQ's standard query operations The DataContext class with features such as entity record monitoring, automatic SQL statement generation, record concurrency detection, and much more
What is a Level of Granularity of a Fact Table?
Level of granularity means the level of detail that you put into the fact table in a data warehouse. Level of granularity implies the detail you are willing to put for each transactional fact.
What is Difference between Line Feed (\n) and Carriage Return (\r)?
Line Feed - LF - \n - 0x0a - 10 (decimal) Carriage Return - CR - \r - 0x0D - 13 (decimal) DECLARE @NewLineChar AS CHAR(2) = CHAR(13) + CHAR(10) PRINT ('SELECT FirstLine AS FL ' +@NewLineChar + 'SELECT SecondLine AS SL' )
What is a Linked Server?
Linked Servers is a concept in SQL Server by which we can add other SQL Server to a Group and query both the SQL Server databases using T-SQL Statements. With a linked server, you can create very clean, easy-to-follow SQL statements that allow remote data to be retrieved, joined and combined with local data. Stored Procedures sp_addlinkedserver, sp_addlinkedsrvlogin will be used to add new Linked Server. (Read more here)
What is Log Shipping?
Log shipping is the process of automating the backup of database and transaction log files on a production SQL server and then restoring them onto a standby server. All Editions (except Express Edition) supports log shipping. In log shipping, the transactional log file from one server is automatically updated into the backup database on the other server. If one server fails, the other server will have the same db and can be used this as the Disaster Recovery plan. The key feature of log shipping is that it will automatically backup transaction logs throughout the day and automatically restore them on the standby server at defined intervals.
Does the Order of Columns in UPDATE statements Matter?
No, the order of columns in UPDATE statement does not matter for results updated. Both the below options produce the same results. Option 1: UPDATE TableName SET Col1 ='Value', Col2 ='Value2' Option 2: UPDATE TableName SET Col2 ='Value2', Col1 ='Value'
What is MERGE Statement?
MERGE is a new feature that provides an efficient way to perform multiple DML operations. In previous versions of SQL Server, we had to write separate statements to INSERT, UPDATE, or DELETE data based on certain conditions, but now, using MERGE statement, we can include the logic of such data modifications in one statement that even checks when the data is matched, then just update it, and when unmatched, insert it. One of the most important advantages of MERGE statement is all the data is read and processed only once.
What is MDS?
Master Data Services or MDS helps enterprises standardize the data people rely on to make critical business decisions. With Master Data Services, IT organizations can centrally manage critical data assets companywide and across diverse systems, enable more people to securely manage master data directly, and ensure the integrity of information over time.
How to Rebuild the Master Database?
Master database is system database and it contains information about running server's configuration. When SQL Server 2005 is installed, it usually creates master, model, msdb, tempdb, resourcedb and the distribution system database by default. Only the Master database is the one which is absolutely a must-have database. Without the Master database, the SQL Server cannot be started. This is the reason why it is extremely important to backup the Master database. To rebuild the Master database, run Setup.exe, verify, and repair a SQL Server instance, and rebuild the system databases. This procedure is most often used to rebuild the master database for a corrupted installation of SQL Server.
Is It Possible to have Clustered Index on Separate Drive From Original Table Location?
No! It is not possible.
If any Stored Procedure is Encrypted, then can we see its definition in Activity Monitor?
No, we can't see definition of encrypted stored procedure in Activity Monitor.
Can we Insert Data if Clustered Index is Disabled?
No, we cannot insert data if Clustered Index is disabled because Clustered Indexes are in fact original tables which are physically ordered according to one or more keys (Columns).
Is OLTP Database is Design Optimal for Data Warehouse?
No. OLTP database tables are normalized, and it will add additional time to queries to return results. Additionally, the OLTP database is small; it does not contain data from a long period (many years), which needs to be analyzed. A OLTP system is basically an ER model and not a Dimensional Model. If a complex query is executed on an OLTP system, it may lead to heavy overhead on the OLTP server that will affect the normal business processes.
What are Non-Additive Facts?
Non-additive facts are facts that cannot be summed up for any of the dimensions present in the fact table. However, they are not considered as useless. If there are changes in dimensions, the same facts can be useful.
What is ODS?
ODS is the abbreviation of Operational Data Store ‑ a database structure that is a repository for near real-time operational data rather than long-term trend data. The ODS may further become the enterprise-shared operational database, allowing operational systems that are being re-engineered to use the ODS as their operation databases.
What is OLAP?
OLAP is abbreviation of Online Analytical Processing. This system is an application that collects, manages, processes and presents multidimensional data for analysis and management purposes.
What is OLTP?
OLTP is abbreviation of On-Line Transaction Processing. This system is an application that modifies data At the very instant it is received and has a large number of concurrent users.
What are Various Limitations of the Views?
ORDER BY clause does not work in View. Regular queries or Stored Procedures give us flexibility when we need another column; we can add a column to regular queries right away. If we want to do the same with Views, then we will have to modify them first. Index created on view not used often. Once the view is created and if the basic table has any column added or removed, it is not usually reflected in the view till it is refreshed. One of the most prominent limitations of the View it is that it does not support COUNT (*); however, it can support COUNT_BIG (*). UNION Operation is now allowed in Indexed View. We cannot create an Index on a nested View situation means we cannot create index on a view which is built from another view. SELF JOIN Not Allowed in Indexed View. Outer Join Not Allowed in Indexed Views. Cross Database Queries Not Allowed in Indexed View.
How to Implement One-to-One, One-to-Many and Many-to-Many Relationships while Designing Tables?
One-to-One relationship can be implemented as a single table and rarely as two tables with primary and foreign key relationships. One-to-Many relationships are implemented by splitting the data into two tables with primary key and foreign key relationships. Many-to-Many relationships are implemented using a junction table with the keys from both the tables forming the composite primary key of the junction table.
What are Pessimistic Lock and Optimistic Lock?
Optimistic Locking is a strategy where you read a record, take note of a version number and check that the version hasn't changed before you write the record back. If the record is dirty (i.e. different version to yours), then you abort the transaction and the user can re-start it. Pessimistic Locking is when you lock the record for your exclusive use until you have finished with it. It has much better integrity than optimistic locking but requires you to be careful with your application design to avoid Deadlocks.
What is Policy Management?
Policy Management in SQL SERVER 2008 allows you to define and enforce policies for configuring and managing SQL Server across the enterprise. Policy-Based Management is configured in SQL Server Management Studio (SSMS). Navigate to the Object Explorer and expand the Management node and the Policy Management node; you will see the Policies, Conditions, and Facets nodes.
What are Primary Keys and Foreign Keys?
Primary keys are the unique identifiers for each row. They must contain unique values and cannot be null. Due to their importance in relational databases, Primary keys are the most fundamental aspect of all keys and constraints. A table can have only one primary key. Foreign keys are a method of ensuring data integrity and manifestation of the relationship between tables.
What are the Properties and Different Types of Sub-Queries?
Properties of a Sub-Query A sub-query must be enclosed in the parenthesis. A sub-query must be put on the right hand of the comparison operator, and A sub-query cannot contain an ORDER BY clause, however sub-query can use ORDER BY when used with TOP clause. Read Comment by David Bridge A query can contain more than one sub-query. Types of Sub-query Single-row sub-query, where the sub-query returns only one row. Multiple-row sub-query, where the sub-query returns multiple rows, and Multiple column sub-query, where the sub-query returns multiple columns
What is Difference between ROLLBACK IMMEDIATE and WITH NO_WAIT during ALTER DATABASE?
ROLLBACK AFTER integer [SECONDS] | ROLLBACK IMMEDIATE: Specifies whether to roll back after a specified number of seconds or immediately if transaction is not complete. NO_WAIT: Specifies that if the requested database state or option change cannot complete immediately without waiting for transactions to commit or roll back on their own, then the request will fail.
What is a ROLLUP Clause?
ROLLUP clause is used to do aggregate operation on multiple levels in hierarchy. If we want sum on different levels without adding any new column, then we can do it easily using ROLLUP. We have to just add the WITH ROLLUP Clause in group by clause.
What is ROW_NUMBER()?
ROW_NUMBER() returns a column as an expression that contains the row's number within the result set. This is only a number used in the context of the result set; if the result changes, the ROW_NUMBER() will change.
What is RAISEERROR?
RaiseError generates an error message and initiates error processing for the session. RAISERROR can either reference a user-defined message stored in the sys.messages catalog view or build a message dynamically. The message is returned as a server error message to the calling application or to an associated CATCH block of a TRY...CATCH construct.
What are Ranking Functions?
Ranking functions return a ranking value for each row in a partition. All the ranking functions are non-deterministic. The different Ranking functions are as follows: ROW_NUMBER () OVER ([<partition_by_clause>] <order_by_clause>) Returns the sequential number of a row within a partition of a result set, starting at 1 for the first row in each partition. RANK () OVER ([<partition_by_clause>] <order_by_clause>) Returns the rank of each row within the partition of a result set. DENSE_RANK () OVER ([<partition_by_clause>] <order_by_clause>) Returns the rank of rows within the partition of a result set, without any gaps in the ranking.
What is RDBMS?
Relational Data Base Management Systems (RDBMS) are database management systems that maintain data records and indices in tables. Relationships may be created and maintained across and among the data and tables. In a relational database, relationships between data items are expressed by means of tables. Interdependencies among these tables are expressed by data values rather than by pointers. This allows a high degree of data independence. An RDBMS has the capability to recombine the data items from different files, providing powerful tools for data usage. (Read more here)
What are the Properties of the Relational Tables?
Relational tables have the following six properties: Values are atomic. Column values are of the same kind. Each row is unique. The sequence of columns is insignificant. The sequence of rows is insignificant. Each column must have a unique name.
Why can't I use Outer Join in an Indexed View?
Rows can logically disappear from an indexed view based on OUTER JOIN when you insert data into a base table. This makes incrementally updating OUTER JOIN views relatively complex to implement, and the performance of the implementation would be slower than for views based on standard (INNER) JOIN.(Read More Here)
How to Find Tables without Indexes?
Run the following query in the Query Editor. USE <database_name>; GO SELECT SCHEMA_NAME(schema_id) AS schema_name ,name AS table_name FROM sys.tables WHERE OBJECTPROPERTY(OBJECT_ID,'IsIndexed') = 0 ORDER BY schema_name, table_name; GO
What are Slowly Changing Dimensions (SCD)?
SCD is the abbreviation of slowly changing dimensions. SCD applies to cases where the attribute for a record varies over time. There are three different types of SCD. SCD1: The new record replaces the original record. Only one record exists in database - current data. SCD2: A new record is added into the customer dimension table. Two records exist in the database - current data and previous history data. SCD3: The original data is modified to include new data. One record exists in database - new information is attached with old information in same row.
Name 3 ways to get an Accurate Count of the Number of Records in a Table?
SELECT * FROM table1 SELECT COUNT(*) FROM table1 SELECT rows FROM sysindexes WHERE id = OBJECT_ID(table1) AND indid < 2
Which Command using Query Analyzer will give you the Version of SQL Server and Operating System?
SELECT SERVERPROPERTY('Edition') AS Edition, SERVERPROPERTY('ProductLevel') AS ProductLevel, SERVERPROPERTY('ProductVersion') AS ProductVersion GO
What is an SQL Profiler?
SQL Profiler is a graphical tool that allows system administrators to monitor events in an instance of Microsoft SQL Server. You can capture and save data about each event to a file or SQL Server table to analyze later. For example, you can monitor a production environment to see which stored procedures are hampering performances by executing very slowly. Use SQL Profiler to monitor only the events in which you are interested. If traces are becoming too large, you can filter them based on the information you want, so that only a subset of the event data is collected. Monitoring too many events adds overhead to the server and the monitoring process and can cause the trace file or trace table to grow very large, especially when the monitoring process takes place over a long period of time.
Explain Few of the New Features of SQL Server 2008 Management Studio
SQL Server 2008 Microsoft has upgraded SSMS with many new features as well as added tons of new functionalities requested by DBAs for long time. A few of the important new features are as follows: IntelliSense for Query Editing Multi Server Query Query Editor Regions Object Explorer Enhancements Activity Monitors
What is the Maximum Size per Database for SQL Server Express?
SQL Server Express supports a maximum size of 4 GB per database, which excludes all the log files. 4 GB is not a very large size; however, if the database is properly designed and the tables are properly arranged in a separate database, this limitation can be resolved to a certain extent.
Can SQL Servers Linked to other Servers like Oracle?
SQL Server can be linked to any server provided it has OLE-DB provider from Microsoft to allow a link, e.g. Oracle has an OLE-DB provider that Microsoft provides to add it as a linked server to the SQL Server group
How will you Handle Error in SQL SERVER 2008?
SQL Server now supports the use of TRY...CATCH constructs for providing rich error handling. TRY...CATCH lets us build error handling at the level we need, in the way we need to by setting a region where if any error occurs, it will break out of the region and head to an error handler. The basic structure is as follows: BEGIN TRY <code> END TRY BEGIN CATCH <code> END CATCH So if any error occurs in the TRY block, then execution is diverted to the CATCH block, and the error can be resolved.
Common Questions Asked Which TCP/IP port does the SQL Server run on? How can it be Changed?
SQL Server runs on port 1433. It can be changed from the Network Utility TCP/IP properties -> Port number, both on client and the server.
What is SQL Injection? How to Protect Against SQL Injection Attack?
SQL injection is an attack in which malicious code is inserted into strings that are later passed to an instance of SQL Server for parsing and execution. Any procedure that constructs SQL statements should be reviewed for injection vulnerabilities because SQL Server will execute all syntactically valid queries that it receives. Even parameterized data can be manipulated by a skilled and determined attacker. Here are few methods which can be used to protect again SQL Injection attack: Use Type-Safe SQL Parameters Use Parameterized Input with Stored Procedures Use the Parameters Collection with Dynamic SQL Filtering Input parameters Use the escape character in LIKE clause Wrapping Parameters with QUOTENAME() and REPLACE()
What are Basics of Policy Management?
SQL server 2008 has introduced a policy management framework, which is the latest technique for SQL server database engine. SQL policy administrator uses SQL Server Management Studio to create policies that can handle entities on the server side like the SQL Server objects and the instance of SQL Server databases. It consists of three components: policy administrators (who create policies), policy management, and explicit administration. Policy-based management in SQL Server assists the database administrators in defining and enforcing policies that tie to database objects and instances. These policies allow the administrator to configure and manage SQL server across the enterprise.
Explain MultiServer Query:
SSMS 2008 has a feature to run a query on different servers from one query editor window. First of all, make sure that you registered all the servers under your registered server. Once they are registered, right click on server group name and click New Query. e.g. for server version information, SELECT SERVERPROPERTY('Edition') AS Edition, SERVERPROPERTY('ProductLevel') AS ProductLevel, SERVERPROPERTY('ProductVersion') AS ProductVersion
What is the STUFF Function and How Does it Differ from the REPLACE Function?
STUFF function is used to overwrite existing characters using this syntax: STUFF (string_expression, start, length, replacement_characters), where string_expression is the string that will have characters substituted, start is the starting position, length is the number of characters in the string that are substituted, and replacement_characters are the new characters interjected into the string. REPLACE function is used to replace existing characters of all occurrences. Using the syntax REPLACE (string_expression, search_string, replacement_string), every incidence of search_string found in the string_expression will be replaced with replacement_string.
What is a Scheduled Job or What is a Scheduled Task?
Scheduled tasks let user automate processes that run on regular or predictable cycles. User can schedule administrative tasks, such as cube processing, to run during times of slow business activity. User can also determine the order in which tasks run by creating job steps within a SQL Server Agent job, e.g. back up database and update statistics of the tables. Job steps give user control over flow of execution. If one job fails, then the user can configure SQL Server Agent to continue to run the remaining tasks or to stop execution.
What is the Difference between Seek Predicate and Predicate?
Seek Predicate is the operation that describes the b-tree portion of the Seek. Predicate is the operation that describes the additional filter using non-key columns. Based on the description, it is very clear that Seek Predicate is better than Predicate as it searches indexes, whereas in Predicate, the search is on non-key a column - which implies that the search is on the data in page, files itself.
What is Service Broker?
Service Broker is a message-queuing technology in SQL Server that allows developers to integrate SQL Server fully into distributed applications. Service Broker is a feature which provides facility to SQL Server to send an asynchronous, transactional message. It allows a database to send a message to another database without waiting for the response; so the application will continue to function if the remote database is temporarily unavailable.
What are Different Types of Locks?
Shared Locks: Used for operations that do not change or update data (read-only operations), such as a SELECT statement. Update Locks: Used on resources that can be updated. It prevents a common form of deadlock that occurs when multiple sessions are reading, locking, and potentially updating resources later. Exclusive Locks: Used for data-modification operations, such as INSERT, UPDATE, or DELETE. It ensures that multiple updates cannot be made to the same resource at the same time. Intent Locks: Used to establish a lock hierarchy. The types of intent locks are as follows: intent shared (IS), intent exclusive (IX), and shared with intent exclusive (SIX). Schema Locks: Used when an operation dependent on the schema of a table is executing. The types of schema locks are schema modification (Sch-M) and schema stability (Sch-S). Bulk Update Locks: Used when bulk-copying data into a table and the TABLOCK hint is specified.
What is Standby Servers? Explain Types of Standby Servers.
Standby Server is a type of server that can be brought online in a situation when Primary Server goes offline and application needs continuous (high) availability of the server. There is always a need to set up a mechanism where data and objects from primary server are moved to secondary (standby) server. This mechanism usually involves the process of moving backup from the primary server to the secondary server using T-SQL scripts. Often, database wizards are used to set up this process. Different types of standby servers are given as follows: 1) Hot Standby: Hot Standby can be achieved in the SQL Server using SQL Server 2005 Enterprise Edition and the later enterprise versions. SQL Server 2005 has introduced Mirroring of database that can be configured for automatic failover in a disaster situation. In the case of synchronous mirroring, the database is replicated to both the servers simultaneously. This is a little expensive but provides the best high availability. In this case, both primary and standby servers have same data all the time. 2) Warm Standby: In Warm Standby, automatic failover is not configured. This is usually set up using Log Shipping or asynchronous mirroring. Sometimes warm standby is lagging by a few minutes or seconds, which results into loss of few latest updates when the primary server fails and secondary server needs to come online. Sometimes a warm standby server that is lagging by a few transactions is brought back to the current state by applying the recent transaction log. 3) Cold Standby: Code Standby servers need to be switched manually, and sometimes all the backups as well as the required OS need to be applied. Cold Standby just physically replaces the previous server. (Read more here)
What is a Star Schema?
Star schema is a type of organizing the tables such that we can retrieve the result from the database quickly in the warehouse environment.
Differences between the Star and Snowflake Schema?
Star schema: A single fact table with N number of dimensions; all dimensions will be linked directly with a fact table. This schema is de-normalized and results in simple join and less complex query as well as faster results. Snow schema: Any dimension with extended dimensions is known as snowflake schema; dimensions maybe interlinked or may have one-to-many relationship with other tables. This schema is normalized, and results in complex join leading very complex query (as well as slower results).
What are the Advantages of Using Stored Procedures?
Stored procedure can reduced network traffic and latency, boosting application performance. Stored procedure execution plans can be reused; they staying cached in SQL Server's memory, reducing server overhead. Stored procedures help promote code reuse. Stored procedures can encapsulate logic. You can change stored procedure code without affecting clients. Stored procedures provide better security to your data.
What is DataWarehousing?
Subject-oriented, which means that the data in the database is organized so that all the data elements relating to the same real-world event or object are linked together; Time-variant, which means that the changes to the data in the database are tracked and recorded so that reports can be produced showing changes over time; Non-volatile, which means that data in the database is never over-written or deleted, once committed, the data is static, read-only, but retained for future reporting. Integrated, which means that the database contains data from most or all of an organization's operational applications, and that this data is made consistent.
What is subquery? Explain the Properties of a Subquery?
Subqueries are often referred to as sub-selects as they allow a SELECT statement to be executed arbitrarily within the body of another SQL statement. A subquery is executed by enclosing it in a set of parentheses. Subqueries are generally used to return a single row as an atomic value although they may be used to compare values against multiple rows with the IN keyword. A subquery is a SELECT statement that is nested within another T-SQL statement. A subquery SELECT statement if executed independently of the T-SQL statement, in which it is nested, will return a resultset. This implies that a subquery SELECT statement can stand alone, and it does not depend on the statement in which it is nested. A subquery SELECT statement can return any number of values and can be found in the column list of a SELECT statement, and FROM, GROUP BY, HAVING, and/or ORDER BY clauses of a T-SQL statement. A subquery can also be used as a parameter to a function call. Basically, a subquery can be used anywhere an expression can be used. (Read more here)
What are Synonyms?
Synonyms give you the ability to provide alternate names for database objects. You can alias object names; for example, using the Employee table as Emp. You can also shorten names. This is especially useful when dealing with three and four part names; for example, shortening server.database.owner.object to object.
What do you mean by TABLESAMPLE?
TABLESAMPLE allows you to extract a sampling of rows from a table in the FROM clause. The rows retrieved are random and they are not in any order. This sampling can be based on a percentage of number of rows. You can use TABLESAMPLE when only a sampling of rows is necessary for the application instead of a full result set.
Which are the New Data Types Introduced in SQL SERVER 2008?
The GEOMETRY Type: The GEOMETRY datatype is a system .NET common language runtime (CLR) datatype in SQL Server. This type represents data in a two-dimensional Euclidean coordinate system. The GEOGRAPHY Type: The GEOGRAPHY datatype's functions are the same as with GEOMETRY. The difference between the two is that when you specify GEOGRAPHY, you are usually specifying points in terms of latitude and longitude. New Date and Time Data types: SQL Server 2008 introduces four new data types related to date and time: DATE, TIME, DATETIMEOFFSET, and DATETIME2. DATE: The new DATE data type just stores the date itself. It is based on the Gregorian calendar and handles years from 1 to 9999. TIME: The new TIME (n) type stores time with a range of 00:00:00.0000000 through 23:59:59.9999999. The precision is allowed with this type. TIME supports seconds down to 100 nanoseconds. The n in TIME(n) defines this level of fractional second precision from 0 to 7 digits of precision. The DATETIMEOFFSET Type: DATETIMEOFFSET (n) is the time-zone-aware version of a datetime datatype. The name will appear less odd when you consider what it really is: a date + time + time-zone offset. The offset is based on how far behind or ahead you are from Coordinated Universal Time (UTC) time. The DATETIME2 Type: It is an extension of the datetime type in earlier versions of SQL Server. This new datatype has a date range covering dates from January 1 of year 1 through December 31 of year 9999. This is a definite improvement over the lower boundary of 1753 of the datetime datatype. DATETIME2 not only includes the larger date range, but also has a timestamp and the same fractional precision that TIME type provides.
What is the Correct Order of the Logical Query Processing Phases?
The correct order of the Logical Query Processing Phases is as follows: 1. FROM 2. ON 3. OUTER 4. WHERE 5. GROUP BY 6. CUBE | ROLLUP 7. HAVING 8. SELECT 9. DISTINCT 10. TOP 11. ORDER BY (Read more here)
What is B-Tree?
The database server uses a B-tree structure to organize index information. B-Tree generally has following types of index pages or nodes: Root node: A root node contains node pointers to only one branch node. Branch nodes: A branch node contains pointers to leaf nodes or other branch nodes, which can be two or more. Leaf nodes: A leaf node contains index items and horizontal pointers to other leaf nodes, which can be many.
What are the Advantages of Policy Management?
The following advantages can be achieved by appropriate administration of policy management system. It interacts with various policies for successful system configuration. It handles the changes in the systems that are the result of configuration against authoring policies. It reduces the cost of ownership with simple elaboration of administration tasks. It detects various compliance issues in SQL Server Management Studio.
How can I Check that whether Automatic Statistic Update is Enabled or not?
The following query can be used to know if Automatic Statistic Update: SELECT is_auto_create_stats_on,is_auto_update_stats_on FROM sys.databases WHERE name ='YOUR DATABASE NAME
If denormalizing improves Data Warehouse Processes, then why is the Fact Table is in the Normal Form?
The foreign keys of facts tables are primary keys of Dimension tables. It is clear that the fact table contains columns which are a primary key to another table that itself make a normal form table.
Questions on SQL SERVER 2008 What are the basic functions for master, msdb, model, tempdb and resource databases?
The master database holds information for all the databases located on the SQL Server instance, and it is the glue that holds the engine together. Because SQL Server cannot start without a functioning master database, you must administer this database with care. The msdb database stores information regarding database backups, SQL Agent information, DTS packages, SQL Server jobs, and some replication information such as for log shipping. The tempdb holds temporary objects such as global and local temporary tables and stored procedures. The model is essentially a template database used in the creation of any new user database created in the instance. The resource Database is a read-only database that contains all the system objects that are included in the SQL Server. SQL Server system objects such as sys.objects are physically persisted in the Resource database, but they logically appear in the sys schema of every database. The Resource database does not contain user data or user metadata.
What is Difference between Commit and Rollback when Used in Transactions?
The usual structure of the TRANSACTION is as follows: BEGIN TRANSACTION Operations COMMIT TRANSACTION or ROLLBACK TRANSACTION When Commit is executed, every statement between BEGIN and COMMIT becomes persistent to database. When Rollback is executed, every statement between BEGIN and ROLLBACK are reverted to the state when BEGIN was executed.
What is the XML Datatype?
The xml data type lets you store XML documents and fragments in a SQL Server database. An XML fragment is an XML instance that has a missing single top-level element. You can create columns and variables of the xml type and store XML instances in them. The xml data type and associated methods help integrate XML into the relational framework of SQL Server.
What are the Fundamental Stages of Data Warehousing?
There are four different fundamental stages of Data Warehousing. Offline Operational Databases: Data warehouses in this initial stage are developed by simply copying the database of an operational system to an off-line server where the processing load of reporting does not impact on the operational system's performance. Offline Data Warehouse: Data warehouses in this stage of evolution are updated on a regular time cycle (usually daily, weekly or monthly) from the operational systems, and the data is stored in an integrated reporting-oriented data structure Real Time Data Warehouse: Data warehouses at this stage are updated on a transaction or event basis, every time an operational system performs a transaction (e.g. an order or a delivery or a booking) Integrated Data Warehouse: Data warehouses at this stage are used to generate activity or transactions that are passed back into the operational systems for use in the daily activity of the organization.
Explain Activity Monitors:
There are four graphs percent; Processor Time, Waiting Tasks, Database I/O, Batch Requests/Sec All the four tabs provide very important information; however, the one which I refer most is "Recent Expensive Queries." Whenever I find my server running slow or having any performance-related issues, my first reaction is to open this tab and see which query is running slow. I usually look at the query with the highest number for Average Duration. The Recent Expensive Queries monitors only show queries which are in the SQL Server cache at that moment.
How to Optimize Stored Procedure Optimization?
There are many tips and tricks for the same. Here are few: Include SET NOCOUNT ON statement. Use schema name with object name. Do not use the prefix "sp_" in the stored procedure name. Use IF EXISTS (SELECT 1) instead of (SELECT *). Use the sp_executesql stored procedure instead of the EXECUTE statement. Try to avoid using SQL Server cursors whenever possible. Keep the Transaction as short as possible. Use TRY-Catch for error handling.
How to Optimize Stored Procedure Optimization?
There are many tips and tricks for the same. Here are few: Include SET NOCOUNT ON statement. Use schema name with object name. Do not use the prefix "sp_" in the stored procedure name. Use IF EXISTS (SELECT 1) instead of (SELECT *). Use the sp_executesql stored procedure instead of the EXECUTE statement. Try to avoid using SQL Server cursors whenever possible. Keep the Transaction as short as possible. Use TRY-Catch for error handling.
How to Copy the Tables, Schema and Views from one SQL Server to Another?
There are multiple ways to do this. "Detach Database" from one server and "Attach Database" to another server. Manually script all the objects using SSMS and run the script on a new server. Use Wizard of SSMS.
How to Copy Data from One Table to Another Table?
There are multiple ways to do this. 1) INSERT INTO SELECT This method is used when table is already created in the database earlier and data have to be inserted into this table from another table. If columns listed in the INSERT clause and SELECT clause are same, listing them is not required. 2) SELECT INTO This method is used when table is not created earlier and it needs to be created when data from one table must be inserted into a newly created table from another table. The new table is created using the same data types as those in selected columns.
What are Wait Types?
There are three types of wait types, namely, Resource Waits. Resource waits occur when a worker requests access to a resource that is not available because that resource is either currently used by another worker or it's not yet available. Queue Waits. Queue waits occur when a worker is idle, waiting for work to be assigned. External Waits. External waits occur when an SQL Server worker is waiting for an external event.
What are the Authentication Modes in SQL Server? How can it be Changed?
There are two authentication modes in SQL Server. Windows Mode Mixed Mode - SQL and Windows To change authentication mode in SQL Server, go to Start -> Programs- > Microsoft SQL Server and click SQL Enterprise Manager to run SQL Enterprise Manager from the Microsoft SQL Server program group. Select the server; then from the Tools menu, select SQL Server Configuration Properties and choose the Security page.
What are the Different Methods of Loading Dimension tables?
There are two different ways to load data in dimension tables. Conventional (Slow): All the constraints and keys are validated against the data before it is loaded; this way data integrity is maintained. Direct (Fast): All the constraints and keys are disabled before the data is loaded. Once data is loaded, it is validated against all the constraints and keys. If data is found invalid or dirty, it is not included in index, and all future processes on this data are skipped.
Which are the Important Points to Note when Multilanguage Data is Stored in a Table?
There are two things to keep in mind while storing unicode data. First, the column must be of unicode data type (nchar, nvarchar, ntext). Second, the value must be prefixed with N while insertion. For example, INSERT INTO table (Hindi_col) values (N'hindi data')
Is there any Performance Difference between IF EXISTS (Select null from table) and IF EXISTS (Select 1 from table)?
There is no performance difference between IF EXISTS (Select null from table) and IF EXISTS (Select 1 from table).
What is the Difference between a HAVING clause and a WHERE clause?
They specify a search condition for a group or an aggregate. But the difference is that HAVING can be used only with the SELECT statement. HAVING is typically used in a GROUP BY clause. When GROUP BY is not used, HAVING behaves like a WHERE clause. Having Clause is basically used only with the GROUP BY function in a query, whereas WHERE Clause is applied to each row before they are part of the GROUP BY function in a query. (Read more here)
When is the use of UPDATE_STATISTICS command?
This command is basically used when a large amount of data is processed. If a large amount of deletions, modifications or Bulk Copy into the tables has occurred, it has to update the indexes to take these changes into account. UPDATE_STATISTICS updates the indexes on these tables accordingly.
How do you Load the Time Dimension?
Time dimensions are usually loaded by a program that loops through all possible dates that may appear in the data. 100 years may be represented in a time dimension, with one row per day.
What Command do we Use to Rename a db, a Table and a Column?
To Rename db sp_renamedb 'oldname' , 'newname If someone is using db it will not accept sp_renmaedb. In that case, first bring db to single user mode using sp_dboptions. Use sp_renamedb to rename the database. Use sp_dboptions to bring the database to multi-user mode. e.g. USE MASTER; GO EXEC sp_dboption AdventureWorks, 'Single User', True GO EXEC sp_renamedb 'AdventureWorks', 'AdventureWorks_New' GO EXEC sp_dboption AdventureWorks, 'Single User', False GO To Rename Table We can change the table name using sp_rename as follows: sp_rename 'oldTableName' 'newTableName' e.g. sp_RENAME 'Table_First', 'Table_Last' GO To rename Column The script for renaming any column is as follows: sp_rename 'TableName.[OldcolumnName]', 'NewColumnName', 'Column' e.g. sp_RENAME 'Table_First.Name', 'NameChange' , 'COLUMN' GO
What are Policy Management Terms?
To have a better grip on the concept of Policy-based management, there are some key terms you need to understand. Target - A type of entity that is appropriately managed by Policy-based management. For example, a table, database and index, to name a few. Facet -A property that can be managed in policy-based management. A clear example of facet is the name of Trigger or the Auto Shrink Property of database. Conditions - Criteria that specifies the state of facet to true or false. For example, you can adjust the state of a facet that gives you clear specifications of all stored procedures in the Schema 'Banking'. Policy - A set of rules specified for the server objects or the properties of database.
What is Connection Pooling and why it is Used?
To minimize the cost of opening and closing connections, ADO.NET uses an optimization technique called connection pooling. The pooler maintains ownership of the physical connection. It manages connections by keeping alive a set of active connections for each given connection configuration. Whenever a user calls Open on a connection, the pooler looks for an available connection in the pool. If a pooled connection is available, it returns it to the caller instead of opening a new connection. When the application calls Close on the connection, the pooler returns it to the pooled set of active connections instead of closing it. Once the connection is returned to the pool, it is ready to be reused on the next Open call.
What are Isolation Levels?
Transactions specify an isolation level that defines the degree to which one transaction must be isolated from resource or data modifications made by other transactions. Isolation levels are described in terms of which concurrency side-effects, such as dirty reads or phantom reads, are allowed. Transaction isolation levels control the following: Whether locks are taken when data is read, and what type of locks are requested. How long the read locks are held. Whether a read operation referencing rows modified by another transaction blocks until the exclusive lock on the row is freed, retrieves the committed version of the row that existed at the time the statement or transaction started, and reads the uncommitted data modification.
What is the Difference between a Function and a Stored Procedure?
UDF can be used in the SQL statements anywhere in the WHERE/HAVING/SELECT section, whereas Stored procedures cannot be. UDFs that return tables can be treated as another rowset. This can be used in JOINs with other tables. Inline UDF's can be thought of as views that take parameters and can be used in JOINs and other Rowset operations.
What is the difference between UNION and UNION ALL?
UNION The UNION command is used to select related information from two tables, much like the JOIN command. However, when using the UNION command all selected columns need to be of the same data type. With UNION, only distinct values are selected. UNION ALL The UNION ALL command is equal to the UNION command, except that UNION ALL selects all values. The difference between UNION and UNION ALL is that UNION ALL will not eliminate duplicate rows, instead it just pulls all rows from all the tables fitting your query specifics and combines them into a table.
What is a Table Called, if it has neither Cluster nor Non-cluster Index? What is it Used for?
Unindexed table or Heap. Microsoft Press Books and Book on Line (BOL) refers it as Heap. A heap is a table that does not have a clustered index and therefore, the pages are not linked by pointers. The IAM pages are the only structures that link the pages in a table together. Unindexed tables are good for fast storing of data. Many times, it is better to drop all the indexes from table and then do bulk of INSERTs and restore those indexes after that.
What are sp_configure Commands and SET Commands?
Use sp_configure to display or change server-level settings. To change the database-level settings, use ALTER DATABASE. To change settings that affect only the current user session, use the SET statement. e.g. sp_CONFIGURE 'show advanced', 0 GO RECONFIGURE GO sp_CONFIGURE GO You can run the following command and check the advanced global configuration settings. sp_CONFIGURE 'show advanced', 1 GO RECONFIGURE GO sp_CONFIGURE GO
What is User-defined Functions? What are the types of User-defined Functions that can be created?
User-defined Functions allow defining its own T-SQL functions that can accept zero or more parameters and return a single scalar data value or a table data type. Different Types of User-Defined Functions created are as follows: Scalar User-defined Function A scalar user-defined function returns one of the scalar data types. Text, ntext, image and timestamp data types are not supported. These are the type of user-defined functions that most developers are used to in other programming languages. Inline Table-Value User-defined Function An Inline table-value user-defined function returns a table data type and is an exceptional alternative to a view as the user-defined function can pass parameters into a T-SQL select command and in essence provide us with a parameterized, non-updateable view of the underlying tables. Multi-Statement Table-Value User-defined Function A multi-statement table-value user-defined function returns a table, and it is also an exceptional alternative to a view as the function can support multiple T-SQL statements to build the final result where the view is limited to a single SELECT statement. Also, the ability to pass parameters into a T-SQL select command or a group of them gives us the capability to in essence create a parameterized, non-updateable view of the data in the underlying tables. Within the create function command, you must define the table structure that is being returned. After creating this type of user-defined function, It can be used in the FROM clause of a T-SQL command unlike the behavior encountered while using a stored procedure which can also return record sets. (Read here for example)
What are the Advantages of Using CTE?
Using CTE improves the readability and enables easy maintenance of complex queries. The query can be divided into separate, simple, and logical building blocks, which can be then used to build more complex CTEs until the final result set is generated. CTE can be defined in functions, stored procedures, triggers or even views. After a CTE is defined, it can be used as a Table or a View and can SELECT, INSERT, UPDATE or DELETE Data.
What is NOLOCK?
Using the NOLOCK query optimizer hint is generally considered a good practice in order to improve concurrency on a busy system. When the NOLOCK hint is included in a SELECT statement, no locks are taken on data when data is read. The result is a Dirty Read, which means that another process could be updating the data at the exact time you are reading it. There are no guarantees that your query will retrieve the most recent data. The advantage to performance is that your reading of data will not block updates from taking place, and updates will not block your reading of data. SELECT statements take Shared (Read) locks. This means that multiple SELECT statements are allowed simultaneous access, but other processes are blocked from modifying the data. The updates will queue until all the reads have completed, and reads requested after the update will wait for the updates to complete. The result to your system is delay (blocking).
What is Difference between Table Aliases and Column Aliases? Do they Affect Performance?
Usually, when the name of the table or column is very long or complicated to write, aliases are used to refer them. e.g. SELECT VeryLongColumnName col1 FROM VeryLongTableName tab1 In the above example, col1 and tab1 are the column alias and table alias, respectively. They do not affect the performance at all.
What is the Difference between VARCHAR and VARCHAR(MAX) Datatypes?
VARCHAR stores variable-length character data whose range varies up to 8000 bytes; varchar(MAX) stores variable-length character data whose range may vary beyond 8000 bytes and till 2 GB. TEXT datatype is going to be deprecated in future versions, and the usage of VARCHAR(MAX) is strongly recommended instead of TEXT datatypes.
What is the difference between CHAR and VARCHAR Datatypes?
VARCHARS are variable length strings with a specified maximum length. If a string is less than the maximum length, then it is stored verbatim without any extra characters, e.g. names and emails. CHARS are fixed-length strings with a specified set length. If a string is less than the set length, then it is padded with extra characters, e.g. phone number and zip codes. For instance, for a column which is declared as VARCHAR(30) and populated with the word 'SQL Server,' only 10 bytes will be stored in it. However, if we have declared the column as CHAR(30) and populated with the word 'SQL Server,' it will still occupy 30 bytes in database.
What is VLDB?
VLDB is abbreviation of Very Large Database. For instance, a one-terabyte database can be considered as a VLDB. Typically, these are decision support systems or transaction processing applications serving a large number of users.
How to Recompile Stored Procedure at Run Time?
We can Recompile Stored Procedure in two ways. Option 1: CREATE PROCEDURE dbo.PersonAge(@MinAge INT, @MaxAge INT) WITH RECOMPILE AS SELECT* FROM dbo.tblPerson WHERE Age <= @MinAge AND Age >= @MaxAge GO Option 2: EXEC dbo.PersonAge65, 70 WITHRECOMPILE We can use RECOMPILE hint with a query and recompile only that particular query. However, if the parameters are used in many statements in the stored procedure and we want to recompile all the statements, then instead of using the RECOMPILE option with all the queries, we have one better option that uses WITH RECOMPILE during stored procedure creation or execution. This method is not recommended for large stored procedures because the recompilation of so many statements may outweigh the benefit of a better execution plan. (Read more here)
How to Delete Duplicate Rows?
We can delete duplicate rows using CTE and ROW_NUMBER () feature of SQL Server 2005 and SQL Server 2008. e.g. WITH CTE (COl1,Col2, DuplicateCount) AS ( SELECT COl1,Col2, ROW_NUMBER() OVER(PARTITION BY COl1,Col2 ORDER BY Col1) AS DuplicateCount FROM DuplicateRcordTable ) DELETE FROM CTE WHERE DuplicateCount >1
How to Find Out the List Schema Name and Table Name for the Database?
We can use following script: SELECT '['+SCHEMA_NAME(schema_id)+'].['+name+']' AS SchemaTable FROM sys.tables
How to Find the List of Fixed Hard Drive and Free Space on Server?
We can use the following Stored Procedure to figure out the number of fixed drives (hard drive) a system has along with free space on each of those drives. EXEC master..xp_fixeddrives
How to Find Index Size for Each Index on Table?
We can use the following query to find the size of index. SELECT * FROM sys.indexes WHERE OBJECT_ID=OBJECT_ID('HumanResources.Shift')
What is the Difference between Update Lock and Exclusive Lock?
When Exclusive Lock is on any process, no other lock can be placed on that row or table. Every other process have to wait till Exclusive Lock completes its tasks. Update Lock is a type of Exclusive Lock, except that it can be placed on the row which already has Shared Lock on it. Update Lock reads the data of the row which has the Shared Lock as soon as the Update Lock is ready to change the data it converts itself to the Exclusive Lock.
What does it mean to have QUOTED_IDENTIFIER ON? What are the Implications of having it OFF?
When SET QUOTED_IDENTIFIER is ON, identifiers can be delimited by double quotation marks, and literals must be delimited by single quotation marks. When SET QUOTED_IDENTIFIER is OFF, identifiers cannot be quoted and must follow all T-SQL rules for identifiers.
What is the Bookmark Lookup and RID Lookup?
When a small number of rows are requested by a query, the SQL Server optimizer will try to use a non-clustered index on the column or columns contained in the WHERE clause to retrieve the data requested by the query. If the query requests data from columns not present in the non-clustered index, then the SQL Server must go back to the data pages to get the data in those columns. Even if the table has a clustered index or not, the query will still have to return to the table or clustered index to retrieve the data. In the above scenario, if table has clustered index, it is called bookmark lookup (or key lookup); if the table does not have clustered index, but a non-clustered index, it is called RID lookup.
When I Delete any Data from a Table, does the SQL Server reduce the size of that table?
When data are deleted from any table, the SQL Server does not reduce the size of the table right away; however, it marks those pages as free pages, showing that they belong to the table. When new data are inserted, they are put into those pages first. Once those pages are filled up, SQL Server will allocate new pages. If you wait for sometime, the background process de-allocates the pages, finally reducing the page size.
Explain Query Editor Regions:
When the T-SQL code is more than hundreds of lines, after a while, it becomes more and more confusing. The regions are defined by the following hierarchy: From first GO command to the next GO command. Statements between BEGIN - END, BEGIN TRY - END TRY, BEGIN CATCH - END CATCH
What is XPath?
XPath uses a set of expressions to select nodes to be processed. The most common expression that you'll use is the location path expression, which returns back a set of nodes called a node set. XPath can use both an unabbreviated and abbreviated syntax. The following is the unabbreviated syntax for a location path: /axisName::nodeTest[predicate]/axisName::nodeTest[predicate]
Can a Stored Procedure call itself or a Recursive Stored Procedure? How many levels of SP nesting is possible?
Yes. As T-SQL supports recursion, you can write stored procedures that call themselves. Recursion can be defined as a method of problem solving wherein the solution is arrived at by repetitively applying it to subsets of the problem. A common application of recursive logic is to perform numeric computations that lend themselves to repetitive evaluation by the same processing steps. Stored procedures are nested when one stored procedure calls another or executes managed code by referencing a CLR routine, type, or aggregate. You can nest stored procedures up to 32 levels. Any reference to managed code from a Transact-SQL stored procedure counts as one level against the 32-level nesting limit. Methods invoked from within managed code do not count against this limit.
How can we Rewrite Sub-Queries into Simple Select Statements or with Joins?
Yes. We can rewrite sub-queries using the Common Table Expression (CTE). A Common Table Expression (CTE) is an expression that can be thought of as a temporary result set which is defined within the execution of a single SQL statement. A CTE is similar to a derived table in that it is not stored as an object and lasts only for the duration of the query. e.g. USE AdventureWorks GO WITH EmployeeDepartment_CTE AS ( SELECT EmployeeID,DepartmentID,ShiftID FROM HumanResources.EmployeeDepartmentHistory ) SELECT ecte.EmployeeId,ed.DepartmentID, ed.Name,ecte.ShiftID FROM HumanResources.Department ed INNER JOIN EmployeeDepartment_CTE ecte ON ecte.DepartmentID = ed.DepartmentID GO
What is Snapshot with Reference to Data Warehouse?
You can disconnect the report from the catalog to which it is attached by saving the report with a snapshot of the data.
