Saturday, June 18, 2011
SQL Basics - Interview Questions Part 1
1. Difference between DCL and DML commands?
DCL commands will commit implicitly where in DML commands we have to commit explicitly.
2. Table, Rowid, Rownum
Table is a database object, which is having more than one column associated with its data type.
Rowid is the unique binary address of the row in a table
Rownum it is a temporary number in a memory and was assigned to each row selected by the statement.
3. Pseudo-Columns
Columns that are not created explicitly by the user and can be used explicitly in queries. The pseudo-Columns are rowid, rownum, currval, nextval, sysdate, and level
4. What is a View?
View is Virtual Table, which hold the data at runtime
5. Difference between ordinary view and complex view?
Simple views can be modified easily and these cant hold the data where as complex views cant be modified directly and complex view can hold the data to modify a complex view we have to use INSTEAD OF TRIGGERS.
6. Forced view
Creating a view on a table, which is not there in the database.
7. Inline view
Inline view is basically a subquery with an alias that you can use like a view inside a SQL statement.
8. What is an INDEX and what are the types?
INDEX is a database object used in oracle to provide quick access to rows.
9. Synonym
Synonym is an alias name for any database object like tables, views, functions, procedures.
10. SELECT statement syntax?
SELECT From
Where Group by
Having
Connect prior
11. What is Constraint? Different Constraints?
Constraints are representators of the columns to enforce data entity and consistency. UNIQUE, NOT NULL, Primary key, foreign key, Check.
12. Difference between Primary key and Unique + Not Null constraints?
Unique + Not Null is a combination of two constraints and we can use more than one Unique + Not Null in any table. Primary Key is a single constraint we can use only one time for a table. It can be a referential key for any column in any table.
13. What is NULL?
Default Value.
14. Dual Table
It is a one row, one column table with value X.
15. Difference between Truncate and Delete?
Truncate will delete all the rows from the table with out any condition. It will commit automatically when it fires Where delete will delete all or specified rows based upon the condition here we have to commit explicitly.
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