Can Multiple Columns Be Used in GROUP BY?

You can use multiple columns in the GROUP BY clause as shown in the following example. It
returns how many employees are having the same salary in each department:
SQL> SELECT department_id, salary, count(*)
FROM employees GROUP BY department_id, salary HAVING count(*) > 1;
DEPARTMENT_ID SALARY COUNT(*)
------------- ---------- ----------
90 17000 2
50 3200 4
50 2200 2
50 3600 2
80 10500 2
80 9000 2
50 2700 2