Question

If yes, the first query below should be correct, but shows error ORA-00979: not a GROUP BY expression.
If not, the third query should also throw error…

 SELECT   POSSIBLE.ENAME, POSSIBLE.DEPTNO, POSSIBLE.SAL
 FROM     EMP POSSIBLE
 WHERE    SAL >
   (SELECT   AVG (SAL)
    FROM     EMP AVERAGE group by possible.deptno having
        POSSIBLE.DEPTNO = AVERAGE.DEPTNO) ;

 SELECT   POSSIBLE.ENAME, POSSIBLE.DEPTno, POSSIBLE.SAL
 FROM     EMP POSSIBLE
 WHERE    SAL >
   (SELECT   AVG (SAL)
    FROM     EMP AVERAGE group by average.deptno having
        POSSIBLE.DEPTno = AVERAGE.DEPTno) ;

 SELECT   POSSIBLE.ENAME, POSSIBLE.DEPTNO, POSSIBLE.SAL
 FROM     EMP POSSIBLE
 WHERE    SAL >
   (SELECT   AVG (SAL)
    FROM     EMP AVERAGE group by possible.deptno) ;

Answer 1

Your first query should use WHERE, not HAVING. A HAVING clause is used to filter based on aggregated (i.e. non-group) values, not to filter the rows involved in the grouping.

Aside from that, you don’t even need grouping in the first query.

SELECT   POSSIBLE.ENAME, POSSIBLE.DEPTNO, POSSIBLE.SAL
 FROM     EMP POSSIBLE
 WHERE    SAL >
   (SELECT   AVG (SAL)
    FROM     EMP AVERAGE WHERE
        POSSIBLE.DEPTNO = AVERAGE.DEPTNO) ;