If a group function is used in the SELECT clause, all other selection fields must be group level fields. Non-group fields can not be mixed with group fields in the SELECT clause. The script below gives you an example of invalid SELECT statements with group and non-gorup selection fields:

SQL> SELECT COUNT(*), department_id FROM departments;

ORA-00937: not a single-group group function

In this example, COUNT(*) is a group field and department_id is a non-group field.