Question

Why is this regarded as incorrect GROUP BY usage?

SELECT Empno, Deptno,
SUM(Sal) "Total"
FROM Emp
GROUP BY Deptno;

The result given in my notes is:

Empno   Deptno Total
------- ------ --------
?       10     8750
?       20     10875
?       30     9400

with the reason of:

All columns in SELECT
list must either appear in
GROUP BY or be
aggregated!

But I’m not too sure what it means…

Answer 1

Since both Empno and Deptno are selected, you need to group by both fields:

SELECT Empno, Deptno, SUM(Sal) "Total"
FROM Emp
GROUP BY Empno, Deptno;

or just select Deptno if Empno isn’t used:

SELECT Deptno, SUM(Sal) "Total"
FROM Emp
GROUP BY Deptno;