SQL
What is the difference between Cluster and Non-Cluster Index?
Clustered index is used for easy retrieval of data from the database by altering the way that the records are stored. Database sorts out rows by the column which is set to be clustered index. A nonclustered index does not alter the way it was stored but creates a complete separate object within the table. It point back to the original table rows after searching.
How can you create an empty table from an existing table?
Example will be - Select * into studentcopy from student where 1=2 Here, we are copying student table to another table with the same structure with no rows copied.
What are all the different types of indexes?
There are three types of indexes -. Unique Index. This indexing does not allow the field to have duplicate values if the column is unique indexed. Unique index can be applied automatically when primary key is defined. Clustered Index. This type of index reorders the physical order of the table and search based on the key values. Each table can have only one clustered index. NonClustered Index. NonClustered Index does not alter the physical order of the table and maintains logical order of data. Each table can have 999 nonclustered indexes.
What is a join?
This is a keyword used to query data from more tables based on the relationship between the fields of the tables. Keys play a major role when JOINs are used.
How to select unique records from a table?
Select unique records from a table by using DISTINCT keyword. Select DISTINCT StudentID, StudentName from Student.
Advantages and Disadvantages of Stored Procedure?
Stored procedure can be used as a modular programming - means create once, store and call for several times whenever required. This supports faster execution instead of executing multiple queries. This reduces network traffic and provides better security to the data. Disadvantage is that it can be executed only in the Database and utilizes more memory in the database server.
What is a foreign key?
A foreign key is one table which can be related to the primary key of another table. Relationship needs to be created between two tables by referencing foreign key with the primary key of another table.
What is a primary key?
A primary key is a combination of fields which uniquely specify a row. This is a special kind of unique key, and it has implicit NOT NULL constraint. It means, Primary key values cannot be NULL.
What is recursive stored procedure?
A stored procedure which calls by itself until it reaches some boundary condition. This recursive function or procedure helps programmers to use the same set of code any number of times.
What is a View?
A view is a virtual table which consists of a subset of data contained in a table. Views are not virtually present, and it takes less space to store. View can have data of one or more tables combined, and it is depending on the relationship.
What is an ALIAS command?
ALIAS name can be given to a table or column. This alias name can be referred in WHERE clause to identify the table or column. Example- Select st.StudentID, Ex.Result from student st, Exam as Ex where st.studentID = Ex. StudentID Here, st refers to alias name for student table and Ex refers to alias name for exam table.
What are all types of user defined functions?
Three types of user defined functions are. Scalar Functions. Inline Table valued functions. Multi statement valued functions. Scalar returns unit, variant defined the return clause. Other two types return table as a return.
What is the difference between the WHERE and HAVING clauses?
When GROUP BY is not used, the WHERE and HAVING clauses are essentially equivalent. However, when GROUP BY is used: The WHERE clause is used to filter records from a result. The filtering occurs before any groupings are made. The HAVING clause is used to filter values from a group (i.e., to check conditions after aggregation into groups has been performed).
What is a query?
A DB query is a code written in order to get the information back from the database. Query can be designed in such a way that it matched with our expectation of the result set. Simply, a question to the Database.
What is a trigger?
A DB trigger is a code or programs that automatically execute with response to some event on a table or view in a database. Mainly, trigger helps to maintain the integrity of the database. Example: When a new student is added to the student database, new records should be created in the related tables like Exam, Score and Attendance tables.
What is Datawarehouse?
Datawarehouse is a central repository of data from multiple sources of information. Those data are consolidated, transformed and made available for the mining and online processing. Warehouse data have a subset of data called Data Marts.
What is Denormalization.
DeNormalization is a technique used to access the data from higher to lower normal forms of database. It is also process of introducing redundancy into a table by incorporating data from the related tables.
What is normalization?
Normalization is the process of minimizing redundancy and dependency by organizing fields and table of a database. The main aim of Normalization is to add, delete or modify field that can be made in a single table.
What is Online Transaction Processing (OLTP)?
Online Transaction Processing or OLTP manages transaction based applications which can be used for data entry and easy retrieval processing of data. This processing makes like easier on simplicity and efficiency. It is faster, more accurate results and expenses with respect to OTLP. Example - Bank Transactions on a daily basis.
What is the difference between TRUNCATE and DROP statements?
TRUNCATE removes all the rows from the table, and it cannot be rolled back. DROP command removes a table from the database and operation cannot be rolled back.
What are all the different normalizations?
The normal forms can be divided into 5 forms, and they are explained below -. First Normal Form (1NF):. This should remove all the duplicate columns from the table. Creation of tables for the related data and identification of unique columns. Second Normal Form (2NF):. Meeting all requirements of the first normal form. Placing the subsets of data in separate tables and Creation of relationships between the tables using primary keys. Third Normal Form (3NF):. This should meet all requirements of 2NF. Removing the columns which are not dependent on primary key constraints. Fourth Normal Form (3NF):. Meeting all the requirements of third normal form and it should not have multi- valued dependencies.
What will be the result of the query below? Explain your answer and provide a version that behaves correctly. select case when null = null then 'Yup' else 'Nope' end as Result;
This query will actually yield "Nope", seeming to imply that null is not equal to itself! The reason for this is that the proper way to compare a value to null in SQL is with the is operator, not with =. Accordingly, the correct version of the above query that yields the expected result (i.e., "Yup") would be as follows: select case when null is null then 'Yup' else 'Nope' end as Result;
How can you select all the even number records from a table? All the odd number records?
To select all the even number records from a table: Select * from table where id % 2 = 0 To select all the odd number records from a table: Select * from table where id % 2 != 0
What are tables and Fields?
A table is a set of data that are organized in a model with Columns and Rows. Columns can be categorized as vertical, and Rows are horizontal. A table has specified number of column called fields but can have any number of rows which is called record. Example:. Table: Employee. Field: Emp ID, Emp Name, Date of Birth. Data: 201456, David, 11/15/1960.
What is collation?
Collation is defined as set of rules that determine how character data can be sorted and compared. This can be used to compare A and, other language characters and also depends on the width of the characters. ASCII value can be used to compare these character data.
How to fetch common records from two tables?
Common records result set can be achieved by - Select studentID from student. <strong>INTERSECT </strong> Select StudentID from Exam
What is a constraint?
Constraint can be used to specify the limit on the data type of table. Constraint can be specified while creating or altering the table statement. Sample of constraint are. NOT NULL. CHECK. DEFAULT. UNIQUE. PRIMARY KEY. FOREIGN KEY.
What is a relationship and what are they?
Database Relationship is defined as the connection between the tables in a database. There are various data basing relationships, and they are as follows:. One to One Relationship. One to Many Relationship. Many to One Relationship. Self-Referencing Relationship.
What is a Database?
Database is nothing but an organized form of data for easy access, storing, retrieval and managing of data. This is also known as structured form of data which can be accessed in many ways. Example: School Management Database, Bank Management Database.
What is DBMS?
A Database Management System (DBMS) is a program that controls creation, maintenance and use of a database. DBMS can be termed as File Manager that manages data in a database rather than saving it in file systems.
What is a unique key?
A Unique key constraint uniquely identified each record in the database. This provides uniqueness for the column or set of columns. A Primary key constraint has automatic unique constraint defined on it. But not, in the case of Unique Key. There can be many unique constraint defined per table, but only one Primary key constraint defined per table.
What is a Cursor?
A database Cursor is a control which enables traversal over the rows or records in the table. This can be viewed as a pointer to one row in a set of rows. Cursor is very much useful for traversing such as retrieval, addition and removal of database records.
What is subquery?
A subquery is a query within another query. The outer query is called as main query, and inner query is called subquery. SubQuery is always executed first, and the result of subquery is passed on to the main query.
List and explain each of the ACID properties that collectively guarantee that database transactions are processed reliably.
ACID (Atomicity, Consistency, Isolation, Durability) is a set of properties that guarantee that database transactions are processed reliably. They are defined as follows: Atomicity. Atomicity requires that each transaction be "all or nothing": if one part of the transaction fails, the entire transaction fails, and the database state is left unchanged. An atomic system must guarantee atomicity in each and every situation, including power failures, errors, and crashes. Consistency. The consistency property ensures that any transaction will bring the database from one valid state to another. Any data written to the database must be valid according to all defined rules, including constraints, cascades, triggers, and any combination thereof. Isolation. The isolation property ensures that the concurrent execution of transactions results in a system state that would be obtained if transactions were executed serially, i.e., one after the other. Providing isolation is the main goal of concurrency control. Depending on concurrency control method (i.e. if it uses strict - as opposed to relaxed - serializability), the effects of an incomplete transaction might not even be visible to another transaction. Durability. Durability means that once a transaction has been committed, it will remain so, even in the event of power loss, crashes, or errors. In a relational database, for instance, once a group of SQL statements execute, the results need to be stored permanently (even if the database crashes immediately thereafter). To defend against power loss, transactions (or their effects) must be recorded in a non-volatile memory.
List and explain the different types of JOIN clauses supported in ANSI-standard SQL.
ANSI-standard SQL specifies five types of JOIN clauses as follows: INNER JOIN (a.k.a. "simple join"): Returns all rows for which there is at least one match in BOTH tables. This is the default type of join if no specific JOIN type is specified. LEFT JOIN (or LEFT OUTER JOIN): Returns all rows from the left table, and the matched rows from the right table; i.e., the results will contain all records from the left table, even if the JOIN condition doesn't find any matching records in the right table. This means that if the ON clause doesn't match any records in the right table, the JOIN will still return a row in the result for that record in the left table, but with NULL in each column from the right table. RIGHT JOIN (or RIGHT OUTER JOIN): Returns all rows from the right table, and the matched rows from the left table. This is the exact opposite of a LEFT JOIN; i.e., the results will contain all records from the right table, even if the JOIN condition doesn't find any matching records in the left table. This means that if the ON clause doesn't match any records in the left table, the JOIN will still return a row in the result for that record in the right table, but with NULL in each column from the left table. FULL JOIN (or FULL OUTER JOIN): Returns all rows for which there is a match in EITHER of the tables. Conceptually, a FULL JOIN combines the effect of applying both a LEFT JOIN and a RIGHT JOIN; i.e., its result set is equivalent to performing a UNION of the results of left and right outer queries. CROSS JOIN: Returns all records where each row from the first table is combined with each row from the second table (i.e., returns the Cartesian product of the sets of rows from the joined tables). Note that a CROSS JOIN can either be specified using the CROSS JOIN syntax ("explicit join notation") or (b) listing the tables in the FROM clause separated by commas without using a WHERE clause to supply join criteria ("implicit join notation").
What are aggregate and scalar functions?
Aggregate functions are used to evaluate mathematical calculation and return single values. This can be calculated from the columns in a table. Scalar functions return a single value based on the input value. Example - Aggregate - max(), count - Calculated with respect to numeric. Scalar - UCASE(), NOW() - Calculated with respect to strings.
Given these contents of the Customers table: Id Name ReferredBy 1 John Doe NULL 2 Jane Smith NULL 3 Anne Jenkins 2 4 Eric Branford NULL 5 Pat Richards 1 6 Alice Barnes 2 Here is a query written to return the list of customers not referred by Jane Smith: SELECT Name FROM Customers WHERE ReferredBy <> 2; What will be the result of the query? Why? What would be a better way to write it?
Although there are 4 customers not referred by Jane Smith (including Jane Smith herself), the query will only return one: Pat Richards. All the customers who were referred by nobody at all (and therefore have NULL in their ReferredBy column) don't show up. But certainly those customers weren't referred by Jane Smith, and certainly NULL is not equal to 2, so why didn't they show up? SQL Server uses three-valued logic, which can be troublesome for programmers accustomed to the more satisfying two-valued logic (TRUE or FALSE) most programming languages use. In most languages, if you were presented with two predicates: ReferredBy = 2 and ReferredBy <> 2, you would expect one of them to be true and one of them to be false, given the same value of ReferredBy. In SQL Server, however, if ReferredBy is NULL, neither of them are true and neither of them are false. Anything compared to NULL evaluates to the third value in three-valued logic: UNKNOWN. The query should be written: SELECT Name FROM Customers WHERE ReferredBy IS NULL OR ReferredBy <> 2 Watch out for the following, though! SELECT Name FROM Customers WHERE ReferredBy = NULL OR ReferredBy <> 2 This will return the same faulty set as the original. Why? We already covered that: Anything compared to NULL evaluates to the third value in the three-valued logic: UNKNOWN. That "anything" includes NULL itself! That's why SQL Server provides the IS NULL and IS NOT NULL operators to specifically check for NULL. Those particular operators will always evaluate to true or false. Even if a candidate doesn't have a great amount of experience with SQL Server, diving into the intricacies of three-valued logic in general can give a good indication of whether they have the ability learn it quickly or whether they will struggle with it.
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's query optimizer for a stored procedure or ad hoc query. Execution plans are very useful for helping a developer understand and analyze the performance characteristics of a query or stored procedure, since the plan is used to execute the query or stored procedure. In many SQL systems, a textual execution plan can be obtained using a keyword such as EXPLAIN, and visual representations can often be obtained as well. In Microsoft SQL Server, the Query Analyzer has an option called "Show Execution Plan" (located on the Query drop down menu). If this option is turned on, it will display query execution plans in a separate window when a query is run.
What is an Index?
An index is performance tuning method of allowing faster retrieval of records from the table. An index creates an entry for each value and it will be faster to retrieve data.
What is Auto Increment?
Auto increment keyword allows the user to create a unique number to be generated when a new record is inserted into the table. AUTO INCREMENT keyword can be used in Oracle and IDENTITY keyword can be used in SQL SERVER. Mostly this keyword can be used whenever PRIMARY KEY is used.
What is the difference between DELETE and TRUNCATE commands?
DELETE command is used to remove rows from the table, and WHERE clause can be used for conditional set of parameters. Commit and Rollback can be performed after delete statement. TRUNCATE removes all rows from the table. Truncate operation cannot be rolled back.
What is data Integrity?
Data Integrity defines the accuracy and consistency of data stored in a database. It can also define integrity constraints to enforce business rules on the data when it is entered into the application or database.
What are all different types of collation sensitivity?
Following are different types of collation sensitivity -. Case Sensitivity - A and a and B and b. Accent Sensitivity. Kana Sensitivity - Japanese Kana characters. Width Sensitivity - Single byte character and double byte character.
Given two tables created as follows create table test_a(id numeric); create table test_b(id numeric); insert into test_a(id) values (10), (20), (30), (40), (50); insert into test_b(id) values (10), (30), (50); Write a query to fetch values in table test_a that are and not in test_b without using the NOT keyword.
In SQL Server, PostgreSQL, and SQLite, this can be done using the except keyword as follows: select * from test_a except select * from test_b; In Oracle, the minus keyword is used instead. MySQL does not support the except function, so it is necessary to use not in.
Which operator is used in query for pattern matching?
LIKE operator is used for pattern matching, and it can be used as -. 1. % - Matches zero or more characters. 2. _(Underscore) - Matching exactly one character. Example - Select * from Student where studentname like 'a%' Select * from Student where studentname like 'ami_'
What are local and global variables and their differences?
Local variables are the variables which can be used or exist inside the function. They are not known to the other functions and those variables cannot be referred or used. Variables can be created whenever that function is called. Global variables are the variables which can be used or exist throughout the program. Same variable declared in global cannot be used in functions. Global variables cannot be created whenever that function is called.
What is RDBMS?
RDBMS stands for Relational Database Management System. RDBMS store the data into the collection of tables, which is related by common fields between the columns of the table. It also provides relational operators to manipulate the data stored into the tables. Example: SQL Server.
How to fetch alternate records from a table?
Records can be fetched for both Odd and Even row numbers - To display even numbers- Select studentId from (Select rowno, studentId from student) where mod(rowno,2)=0 To display odd numbers- Select studentId from (Select rowno, studentId from student) where mod(rowno,2)=1 from (Select rowno, studentId from student) where mod(rowno,2)=1.[/sql]
Given the following tables: SELECT * FROM users; user_id username 1 John Doe 2 Jane Don 3 Alice Jones 4 Lisa Romero SELECT * FROM training_details; user_training_id user_id training_id training_date 1 1 1 "2015-08-02" 2 2 1 "2015-08-03" 3 3 2 "2015-08-02" 4 4 2 "2015-08-04" 5 2 2 "2015-08-03" 6 1 1 "2015-08-02" 7 3 2 "2015-08-04" 8 4 3 "2015-08-03" 9 1 4 "2015-08-03" 10 3 1 "2015-08-02" 11 4 2 "2015-08-04" 12 3 2 "2015-08-02" 13 1 1 "2015-08-02" 14 4 3 "2015-08-03" Write a query to to get the list of users who took the a training lesson more than once in the same day, grouped by user and training lesson, each ordered from the most recent lesson date to oldest date.
SELECT u.user_id, username, training_id, training_date, count( user_training_id ) AS count FROM users u JOIN training_details t ON t.user_id = u.user_id GROUP BY user_id, training_id, training_date HAVING count( user_training_id ) > 1 ORDER BY training_date DESC;
Given a table dbo.users where the column user_id is a unique identifier, how can you efficiently select the first 100 odd user_id values from the table? (Assume the table contains well over 100 records with odd user_id values.)
SELECT TOP 100 user_id FROM dbo.users WHERE user_id % 2 = 1 ORDER BY user_id
What is CLAUSE?
SQL clause is defined to limit the result set by providing condition to the query. This usually filters some rows from the whole set of records. Example - Query that has WHERE condition Query that has HAVING condition.
What is SQL?
SQL stands for Structured Query Language , and it is used to communicate with the Database. This is a standard language used to perform tasks such as retrieval, updation, insertion and deletion of data from a database. Standard SQL Commands are Select.
What is Self-Join?
Self-join is set to be query used to compare to itself. This is used to compare values in a column with other values in the same column in the same table. ALIAS ES can be used for the same table comparison.
What is a stored procedure?
Stored Procedure is a function consists of many SQL statement to access the database system. Several SQL statements are consolidated into a stored procedure and execute them whenever and wherever required.
Given the following tables: sql> SELECT * FROM runners; +----+--------------+ | id | name | +----+--------------+ | 1 | John Doe | | 2 | Jane Doe | | 3 | Alice Jones | | 4 | Bobby Louis | | 5 | Lisa Romero | +----+--------------+ sql> SELECT * FROM races; +----+----------------+-----------+ | id | event | winner_id | +----+----------------+-----------+ | 1 | 100 meter dash | 2 | | 2 | 500 meter dash | 3 | | 3 | cross-country | 2 | | 4 | triathalon | NULL | +----+----------------+-----------+ What will be the result of the query below? SELECT * FROM runners WHERE id NOT IN (SELECT winner_id FROM races) Explain your answer and also provide an alternative version of this query that will avoid the issue that it exposes.
Surprisingly, given the sample data provided, the result of this query will be an empty set. The reason for this is as follows: If the set being evaluated by the SQL NOT IN condition contains any values that are null, then the outer query here will return an empty set, even if there are many runner ids that match winner_ids in the races table. Knowing this, a query that avoids this issue would be as follows: SELECT * FROM runners WHERE id NOT IN (SELECT winner_id FROM races WHERE winner_id IS NOT null)
What is wrong with this SQL query? Correct it so it executes properly. SELECT Id, YEAR(BillingDate) AS BillingYear FROM Invoices WHERE BillingYear >= 2010;
The expression BillingYear in the WHERE clause is invalid. Even though it is defined as an alias in the SELECT phrase, which appears before the WHERE phrase, the logical processing order of the phrases of the statement is different from the written order. Most programmers are accustomed to code statements being processed generally top-to-bottom or left-to-right, but T-SQL processes phrases in a different order. The correct query should be: SELECT Id, YEAR(BillingDate) AS BillingYear FROM Invoices WHERE YEAR(BillingDate) >= 2010;
Consider the following two query results: SELECT count(*) AS total FROM orders; +-------+ | total | +-------+ | 100 | +-------+ SELECT count(*) AS cust_123_total FROM orders WHERE customer_id = '123'; +----------------+ | cust_123_total | +----------------+ | 15 | +----------------+ Given the above query results, what will be the result of the query below? SELECT count(*) AS cust_not_123_total FROM orders WHERE customer_id <> '123'
The obvious answer is 85 (i.e, 100 - 15). However, that is not necessarily correct. Specifically, any records with a customer_id of NULL will not be included in either count (i.e., they won't be included in cust_123_total, nor will they be included in cust_not_123_total). For example, if exactly one of the 100 customers has a NULL customer_id, the result of the last query will be: +--------- ----------+ | cust_not_123_total | +--------------------+ | 84 | +--------------------+
What is the difference between the RANK() and DENSE_RANK() functions? Provide an example.
The only difference between the RANK() and DENSE_RANK() functions is in cases where there is a "tie"; i.e., in cases where multiple values in a set have the same ranking. In such cases, RANK() will assign non-consecutive "ranks" to the values in the set (resulting in gaps between the integer ranking values when there is a tie), whereas DENSE_RANK() will assign consecutive ranks to the values in the set (so there will be no gaps between the integer ranking values in the case of a tie). For example, consider the set {25, 25, 50, 75, 75, 100}. For such a set, RANK() will return {1, 1, 3, 4, 4, 6} (note that the values 2 and 5 are skipped), whereas DENSE_RANK() will return {1,1,2,3,3,4}.
Assume a schema of Emp ( Id, Name, DeptId ) , Dept ( Id, Name). If there are 10 records in the Emp table and 5 records in the Dept table, how many rows will be displayed in the result of the following SQL query: Select * From Emp, Dept Explain your answer.
The query will result in 50 rows as a "cartesian product" or "cross join", which is the default whenever the 'where' clause is omitted.
Given two tables created and populated as follows: CREATE TABLE dbo.envelope(id int, user_id int); CREATE TABLE dbo.docs(idnum int, pageseq int, doctext varchar(100)); INSERT INTO dbo.envelope VALUES (1,1), (2,2), (3,3); INSERT INTO dbo.docs(idnum,pageseq) VALUES (1,5), (2,6), (null,0); What will the result be from the following query: UPDATE docs SET doctext=pageseq FROM docs INNER JOIN envelope ON envelope.id=docs.idnum WHERE EXISTS ( SELECT 1 FROM dbo.docs WHERE id=envelope.id ); Explain your answer.
The result of the query will be as follows: idnum pageseq doctext 1 5 5 2 6 6 NULL 0 NULL The EXISTS clause in the above query is a red herring. It will always be true since ID is not a member of dbo.docs. As such, it will refer to the envelope table comparing itself to itself! The idnum value of NULL will not be set since the join of NULL will not return a result when attempting a match with any value of envelope.
What is the command used to fetch first 5 characters of the string?
There are many ways to fetch first 5 characters of the string - Select SUBSTRING(StudentName,1,5) as studentname from student Select RIGHT(Studentname,5) as studentname from student
What are the types of subquery?
There are two types of subquery - Correlated and Non-Correlated. A correlated subquery cannot be considered as independent query, but it can refer the column in a table listed in the FROM the list of the main query. A Non-Correlated sub query can be considered as independent query and the output of subquery are substituted in the main query.
What are the types of join and explain each?
There are various types of join which can be used to retrieve data and it depends on the relationship between tables. Inner join. Inner join return rows when there is at least one match of rows between the tables. Right Join. Right join return rows which are common between the tables and all rows of Right hand side table. Simply, it returns all the rows from the right hand side table even though there are no matches in the left hand side table. Left Join. Left join return rows which are common between the tables and all rows of Left hand side table. Simply, it returns all the rows from Left hand side table even though there are no matches in the Right hand side table. Full Join. Full join return rows when there are matching rows in any one of the tables. This means, it returns all the rows from the left hand side table and all the rows from the right hand side table.
Write a SQL query to find the 10th highest employee salary from an Employee table. Explain your answer. (Note: You may assume that there are at least 10 records in the Employee table.)
This can be done as follows: SELECT TOP (1) Salary FROM ( SELECT DISTINCT TOP (10) Salary FROM Employee ORDER BY Salary DESC ) AS Emp ORDER BY Salary This works as follows: First, the SELECT DISTINCT TOP (10) Salary FROM Employee ORDER BY Salary DESC query will select the top 10 salaried employees in the table. However, those salaries will be listed in descending order. That was necessary for the first query to work, but now picking the top 1 from that list will give you the highest salary not the the 10th highest salary. Therefore, the second query reorders the 10 records in ascending order (which the default sort order) and then selects the top record (which will now be the lowest of those 10 salaries). Not all databases support the TOP keyword. For example, MySQL and PostreSQL use the LIMIT keyword, as follows: SELECT Salary FROM (SELECT DISTINCT Salary FROM Employee ORDER BY Salary DESC LIMIT 10) AS Emp ORDER BY Salary LIMIT 1;
Given a table TBL with a field Nmbr that has rows with the following values: 1, 0, 0, 1, 1, 1, 1, 0, 0, 1, 0, 1, 0, 1, 0, 1 Write a query to add 2 where Nmbr is 0 and add 3 where Nmbr is 1.
This can be done as follows: update TBL set Nmbr = case when Nmbr > 0 then Nmbr+3 else Nmbr+2 end;
What is a key difference between Truncate and Delete?
Truncate is used to delete table content and the action can not be rolled back, whereas Delete is used to delete one or more rows in the table and can be rolled back.
What does UNION do? What is the difference between UNION and UNION ALL?
UNION merges the contents of two structurally-compatible tables into a single combined table. The difference between UNION and UNION ALL is that UNION will omit duplicate records whereas UNION ALL will include duplicate records. It is important to note that the performance of UNION ALL will typically be better than UNION, since UNION requires the server to do the additional work of removing any duplicates. So, in cases where is is certain that there will not be any duplicates, or where having duplicates is not a problem, use of UNION ALL would be recommended for performance reasons.
What is Union, minus and Interact commands?
UNION operator is used to combine the results of two tables, and it eliminates duplicate rows from the tables. MINUS operator is used to return rows from the first query but not from the second query. Matching records of first and second query and other rows from the first query will be displayed as a result set. INTERSECT operator is used to return rows returned by both the queries.
Given a table SALARIES, such as the one below, that has m = male and f = female values. Swap all f and m values (i.e., change all f values to m and vice versa) with a single update query and no intermediate temp table. Id Name Sex Salary 1 A m 2500 2 B f 1500 3 C m 5500 4 D f 500
UPDATE SALARIES SET sex = CASE sex WHEN 'm' THEN 'f' ELSE 'm' END
What is user defined functions?
User defined functions are the functions written to use that logic whenever required. It is not necessary to write the same logic several times. Instead, function can be called or executed whenever needed.
Write a SQL query using UNION ALL (not UNION) that uses the WHERE clause to eliminate duplicates. Why might you want to do this?
You can avoid duplicates using UNION ALL and still run much faster than UNION DISTINCT (which is actually same as UNION) by running a query like this: SELECT * FROM mytable WHERE a=X UNION ALL SELECT * FROM mytable WHERE b=Y AND a!=X The key is the AND a!=X part. This gives you the benefits of the UNION (a.k.a., UNION DISTINCT) command, while avoiding much of its performance hit.
What are the NVL and the NVL2 functions in SQL? How do they differ?
Both the NVL(exp1, exp2) and NVL2(exp1, exp2, exp3) functions check the value exp1 to see if it is null. With the NVL(exp1, exp2) function, if exp1 is not null, then the value of exp1 is returned; otherwise, the value of exp2 is returned, but case to the same data type as that of exp1. With the NVL2(exp1, exp2, exp3) function, if exp1 is not null, then exp2 is returned; otherwise, the value of exp3 is returned.
What is Cross-Join?
Cross join defines as Cartesian product where number of rows in the first table multiplied by number of rows in the second table. If suppose, WHERE clause is used in cross join then the query will work like an INNER JOIN.
