21. 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.
22. 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.
23. 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.
24. 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.
25. 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.
26. 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.
- PRIMARY KEY.
- FOREIGN KEY.
27. 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.
28. 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.
29. 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.
30. 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.