ORA-00979:не е израз GROUP BY, когато изпълнявам своя sql

Проблемът е, че поръчвате по нещо, което не е във вашата group by клауза.

Например, това работи

SQL> with testGroup as ( select 1 as one, 2 as two from dual)
  2  select one
  3  from testGroup
  4  group by one;

       ONE
----------
         1

Ако order by колона, която не е във вашата group by клауза:

SQL> with testGroup as ( select 1 as one, 2 as two from dual)
  2  select one
  3  from testGroup
  4  group by two;
select one
       *
ERROR at line 2:
ORA-00979: not a GROUP BY expression

Ако редактирате group by клауза за обработка на колоната, от която се нуждаете в order by :

SQL> with testGroup as ( select 1 as one, 2 as two from dual)
  2  select one
  3  from testGroup
  4  group by one, two;

       ONE
----------
         1

SQL>