You can divide query output into multiple groups with the GROUP BY clause. It allows you specify a column as the grouping criteria, so that rows with the same value in the column will be considered as a single group. When the GROUP BY clause is specified, the select statement can only be used to return group level information. The following script gives you a good GROUP BY example:

SQL> SELECT department_id, MIN(salary), MAX(salary),

2 AVG(salary) FROM employees GROUP BY department_id;

DEPARTMENT_ID MIN(SALARY) MAX(SALARY) AVG(SALARY)

------------- ----------- ----------- -----------

  100  6900  12000  8600

   30  2500  11000  4150

    7000  7000  7000

   90  17000  24000 19333.3333

   20  6000  13000  9500

   70  10000  10000  10000

  110  8300  12000  10150

   50  2100  8200 3475.55556

......