How To Divide Query Output into Groups? - Oracle DBA FAQ - Understanding SQL SELECT Query Statements

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
......