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.