Subquery in GROUP BY CLAUSE

Question

Is it possible to place a subquery in the GROUP BY CLAUSE? If it is, can anyone show me an example? thanks

Answer 1

Hi,
Here's a GROUP BY clause that uses an IN-subquery:
SELECT    COUNT (*)     AS cnt
FROM       scott.emp
GROUP BY  CASE
          WHEN deptno  IN (
                         SELECT     deptno
                         FROM     scott.dept
                         WHERE     loc     = 'NEW YORK'
          THEN  1
          ELSE  2
       END
;I've never seen a need for this (and I've been to a state fair, a rodeo and a picnic!)
Interestingly, if I copy the expression from the GROUP BY clause to the SELECT clause:
SELECT    COUNT (*)     AS cnt
,       CASE
          WHEN deptno  IN (
                         SELECT     deptno
                         FROM     scott.dept
                         WHERE     loc     = 'NEW YORK'
          THEN  1
          ELSE  2
       END          AS new_york
FROM       scott.emp
GROUP BY  CASE
          WHEN deptno  IN (
                         SELECT     deptno
                         FROM     scott.dept
                         WHERE     loc     = 'NEW YORK'
          THEN  1
          ELSE  2
       END
;I get this error:
...             WHEN deptno  IN (
ERROR at line 3:
ORA-00979: not a GROUP BY expressionDespite what that book says, you can use an IN-subquery like this in an ORDER BY clause:
SELECT       deptno
FROM       scott.emp
ORDER BY  CASE
          WHEN deptno  IN (
                         SELECT     deptno
                         FROM     scott.dept
                         WHERE     loc     = 'DALLAS'
          THEN  1
          ELSE  2
       END
;(New York is department 10, the first department anyway, so it's not as clear to use the exact same example.)