SQL Interview Questions

1. The most important DDL statements in SQL are:

CREATE TABLE - creates a new database table
ALTER TABLE - alters (changes) a database table
DROP TABLE - deletes a database table
CREATE INDEX - creates an index (search key)
DROP INDEX - deletes an index

2. Operators used in SELECT statements.

= Equal
<> or != Not equal
> Greater than
< Less than
>= Greater than or equal
<= Less than or equal
BETWEEN Between an inclusive range
LIKE Search for a pattern

3. SELECT statements:

SELECT column_name(s) FROM table_name
SELECT DISTINCT column_name(s) FROM table_name
SELECT column FROM table WHERE column operator value
SELECT column FROM table WHERE column LIKE pattern
SELECT column,SUM(column) FROM table GROUP BY column
SELECT column,SUM(column) FROM table GROUP BY column HAVING SUM(column) condition value
Note that single quotes around text values and numeric values should not be enclosed in quotes. Double quotes may be acceptable in some databases.

4. The SELECT INTO Statement is most often used to create backup copies of tables or for archiving records.

SELECT column_name(s) INTO newtable [IN externaldatabase] FROM source
SELECT column_name(s) INTO newtable [IN externaldatabase] FROM source WHERE column_name operator value

5. The INSERT INTO Statements:
INSERT INTO table_name VALUES (value1, value2,....)
INSERT INTO table_name (column1, column2,...) VALUES (value1, value2,....)

6. The Update Statement:

UPDATE table_name SET column_name = new_value WHERE column_name = some_value

7. The Delete Statements:

DELETE FROM table_name WHERE column_name = some_value
Delete All Rows:
DELETE FROM table_name or DELETE * FROM table_name
8. Sort the Rows:

SELECT column1, column2, ... FROM table_name ORDER BY columnX, columnY, ..
SELECT column1, column2, ... FROM table_name ORDER BY columnX DESC
SELECT column1, column2, ... FROM table_name ORDER BY columnX DESC, columnY ASC

9. The IN operator may be used if you know the exact value you want to return for at least one of the columns.

SELECT column_name FROM table_name WHERE column_name IN (value1,value2,..)

10. BETWEEN ... AND

SELECT column_name FROM table_name WHERE column_name BETWEEN value1 AND value2 The values can be numbers, text, or dates.

11. What is the use of CASCADE CONSTRAINTS?

When this clause is used with the DROP command, a parent table can be dropped even when a child table exists.

12. Why does the following command give a compilation error?

DROP TABLE &TABLE_NAME; Variable names should start with an alphabet. Here the table name starts with an '&' symbol.

13. Which system tables contain information on privileges granted and privileges obtained?

USER_TAB_PRIVS_MADE, USER_TAB_PRIVS_RECD

14. Which system table contains information on constraints on all the tables created?obtained?

USER_CONSTRAINTS.

15. What is the difference between TRUNCATE and DELETE commands?

16. State true or false. !=, <>, ^= all denote the same operation?

True.

17. State true or false. EXISTS, SOME, ANY are operators in SQL?

True.

18. What will be the output of the following query?

SELECT REPLACE(TRANSLATE(LTRIM(RTRIM('!! ATHEN !!','!'), '!'), 'AN', '**'),'*','TROUBLE') FROM DUAL;?
TROUBLETHETROUBLE.

19. What does the following query do?

SELECT SAL + NVL(COMM,0) FROM EMP;?
This displays the total salary of all employees. The null values in the commission column will be replaced by 0 and added to salary.

20. What is the advantage of specifying WITH GRANT OPTION in the GRANT command?

The privilege receiver can further grant the privileges he/she has obtained from the owner to any other user.

21. Which command executes the contents of a specified file?

START or @.

22. What is the value of comm and sal after executing the following query if the initial value of ‘sal’ is 10000
UPDATE EMP SET SAL = SAL + 1000, COMM = SAL*0.1;?

sal = 11000, comm = 1000.

23. Which command displays the SQL command in the SQL buffer, and then executes it?

RUN.

24. What command is used to get back the privileges offered by the GRANT command?

REVOKE.

25. What will be the output of the following query? SELECT DECODE(TRANSLATE('A','1234567890','1111111111'), '1','YES', 'NO' );? NO.

Explanation : The query checks whether a given string is a numerical digit.

26. Which date function is used to find the difference between two dates?

MONTHS_BETWEEN.

27. What operator performs pattern matching?

LIKE operator.

28. What is the use of the DROP option in the ALTER TABLE command?

It is used to drop constraints specified on the table.

29. What operator tests column for the absence of data?

IS NULL operator.

30. What are the privileges that can be granted on a table by a user to others?

Insert, update, delete, select, references, index, execute, alter, all.

31. Which function is used to find the largest integer less than or equal to a specific value?

FLOOR.

32. Which is the subset of SQL commands used to manipulate Oracle Database structures, including tables?

Data Definition Language (DDL).

33. What is the use of DESC in SQL?

DESC has two purposes. It is used to describe a schema as well as to retrieve rows from table in descending order.
Explanation :
The query SELECT * FROM EMP ORDER BY ENAME DESC will display the output sorted on ENAME in descending order.

34. What command is used to create a table by copying the structure of another table?

CREATE TABLE .. AS SELECT command
Explanation:
To copy only the structure, the WHERE clause of the SELECT command should contain a FALSE statement as in the following.
CREATE TABLE NEWTABLE AS SELECT * FROM EXISTINGTABLE WHERE 1=2;
If the WHERE condition is true, then all the rows or rows satisfying the condition will be copied to the new table.

35. TRUNCATE TABLE EMP;
DELETE FROM EMP;
Will the outputs of the above two commands differ?

Both will result in deleting all the rows in the table EMP..

36. What is the output of the following query SELECT TRUNC(1234.5678,-2) FROM DUAL;?

1200.

37. What are the wildcards used for pattern matching.?

_ for single character substitution and % for multi-character substitution.

38. What is the parameter substitution symbol used with INSERT INTO command?

&