MS SQL SERVER Frequently Asked Interview Questions

What is 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

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 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. 2.A global temporary table remains in the database permanently, but the rows exist only within a given connection. When connection is closed, the data in the global temporary table disappears. However, the table definition remains with the database for access when database is opened next time.

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 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 are the Different Index Configurations a Table can have?

A table can have one of the following indexes 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.

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.

When should you back up user databases?

After Created databases after creating indexes after clearing the tran log after performing nonlogging operations (Backup with truncate only, write text update test select ...)

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 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. From within Query Analyzer is an option called "Show Execution Plan" (located on the Query drop-down menu). If this option is turned on it will display query execution plan in separate window when 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's the Difference between a Primary Key and a Unique Key?

Both primary key and unique key enforces 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.

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.

Facts About 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 the table. •DELETE can be rolled back. •DELETE is DML Command. •DELETE does not reset the identity of the table.

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 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.

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.

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.

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 the main activity monitoring tool for Microsoft sql server

Manzoor

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.

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.

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

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.

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 a Scheduled Jobs or What is a Scheduled Tasks?

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, Update Stats of Tables. Job steps give user control over flow of execution. If one job fails, user can configure SQL Server Agent to continue to run the remaining tasks or to stop execution.

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.

Facts about 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.

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 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

What is the Difference between DELETE and TRUNCATE Commands?

The 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.

what is the sp_addumpdevice?

The procedure used for backups

How to Copy the Tables, Schema and Views from one SQL Server to Another?

There are multiple ways to do this. 1."Detach Database" from one server and "Attach Database" to another server. 2.Manually script all the objects using SSMS and run the script on a new server. 3.Use Wizard of SSMS.

What are the Different Types of Triggers?

There are two 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.

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.

When should you backup the system databases?

after modifying them

In MSSQL SERVER pages are equivalent to..

an oracle data block

What can't you do during a backup?

create or modifying databases performing auto-grow operations creating indexes performing nonlogged operations shrinking a database

What are the system databases in MSSQL SERVER

master model msdb tempdb

What does the Maintenance Plans- Wizard do?

to create task

What are Linked Servers?

used to connect from sql server to another database engine or any other rdbms.

Want more visual based question?

Go to this site: https://www.toptal.com/sql/interview-questions

The main tool for configuration of database in MSSERVER is...

MS SQL SERVER MANAGMENT STUDIO

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 the 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.

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 are the SQL SERVER RECOVERY MODELs?

Full Recovery Model Bulk_Logged Recovery Model Simple Recovery Model

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 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.

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.

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.

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

what does a backup statement do to a transaction log?

it clears it unless you add (no_truncate?)