How to write a query for the given scenario ?

Question

Hi All ,
I am having two tables EMP, DEPT with the below data.
EMP TABLE :--
EID     ENAME     JOB     SAL     DEPID
111     RAM     MANAGER     1500     10
222     SAM     ASST MANAGER     2000     20
333     KALA     CLERK     2500     10
444     BIMA     MANAGER     3000     20
555     CHALA     MANAGER     3500     30
666     RANI     ASST MANAGER     4000     10
777     KAMAL     MANAGER     2400     10
DEPT TABLE :--
DEPID     DNAME
10     XX
20     YY
30     ZZ
Q1 : I want the sum of salary of each department and for the particular job . Here in each departmant manager, asst. manager, clerk posts are there .
I want to display the result like below ....
JOB     10     20     30
MANAGER     3900     3000     3500
ASST MANAGER 4000     2000     NULL
CLERK     2500     NULL     NULL
please tell me how to write a sql query ?
Thanks
Sai

Answer 1

In general case, you cannot write this query.
This is one of the limits of relational database concepts. The number of columns must be known up-front. In the SELECT clause, you have to list and name all columns returned by the query. So you have to know number of departments. (There are some workarounds - you can return one column with concatenated values for all departments, separated by space character).
If you know that you have 3 departments then you qurey will return 4 columns:
SELECT
   e.job,
   SUM ( CASE WHEN d.deptid = 10 THEN e.sal ELSE NULL END) d10,
   SUM ( CASE WHEN d.deptid = 20 THEN e.sal ELSE NULL END) d20,
   SUM ( CASE WHEN d.deptid = 30 THEN e.sal ELSE NULL END) d30
FROM dept d, emp e
WHERE d.deptno = e.deptno
GROUP BY e.job