一个简单的问题

select * from emp 
        empno   ename   job     mgr     hiredate        sal     comm    deptno        
1 7369 SMITH CLERK 7902 1980-12-17 800.00 20
2 7499 ALLEN SALESMAN7698 1981-2-20 1600.00 300.00 30
3 7521 WARD SALESMAN7698 1981-2-22 1250.00 500.00 30
4 7566 JONES MANAGER 7839 1981-4-2 2975.00 20
5 7654 MARTIN SALESMAN7698 1981-9-28 1250.00 1400.00 30
6 7698 BLAKE MANAGER 7839 1981-5-1 2850.00 30
7 7782 CLARK MANAGER 7839 1981-6-9 2450.00 10
8 7788 SCOTT ANALYST 7566 1987-4-19 3000.00 20
9 7839 KING PRESIDENT 1981-11-17 5000.00 10
10 7844 TURNER SALESMAN7698 1981-9-8 1500.00 0.00 30
11 7876 ADAMS CLERK 7788 1987-5-23 1100.00 20
12 7900 JAMES CLERK 7698 1981-12-3 950.00 30
13 7902 FORD ANALYST 7566 1981-12-3 3000.00 20
14 7934 MILLER CLERK 7782 1982-1-23 1300.00 10想从先选择job值相同sal相加取平均值并返回，即：
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job             平均工资
CLERK           1037.50
SALESMAN        1400.00
MANAGER         2758.33
ANALYST         3000.00
PRESIDENT       5000.00