Group Function Is Not Allowed Here

INPUT:
SQL> select count(last_name), first_name, phone_number
2 from employee_tbl
3 group by count(last_name), first_name, phone_number
4 /

OUTPUT:
group by count(last_name), first_name, phone_number
*
ERROR at line 3:
ORA-00934: group function is not allowed here
SQL>

ANALYSIS:
As with any group function, COUNT may not be used in the GROUP BY clause. You can list only column and nongroup functions, such as SUBSTR, in the GROUP BY clause. TIP: COUNT is a function that is being performed on groups in the query. The previous statement has been corrected using the proper syntax:

SQL> select count(last_name), first_name, phone_number
2 from employee_tbl
3 group by last_name, first_name, phone_number;