How to use Group By clause in a query

Example

ID	MANAGER_ID	SAL	DEPT_ID
1	8	3000	20
2	8	3000	20
3	1	800	20
4	7	950	30
5	2	1100	20
6	9	1300	10
7	10	2850	30
8	10	2975	20
9	10	2450	10
10	NULL	5000	10

Count the number of employees group by department

SELECT COUNT(ID) EMP_NO, DEPT_ID FROM EMPLOYEES WHERE MANAGER_ID IS NOT NULL GROUP BY DEPT_ID;

Select the sum of salary group by department

SELECT DEPT_ID, SUM(SAL) SAL_SUM FROM EMPLOYEES GROUP BY DEPT_ID;