mysql入门(分组查询八)

1:单行处理

 1：sum求和

mysql> select sum(sal) from emp;

+----------+

| sum(sal) |

+----------+

| 29025.00 |

+----------+

1 row in set (0.00 sec)

2：avg:求平均值

mysql> select avg(sal) from emp;

+-------------+

| avg(sal)    |

+-------------+

| 2073.214286 |

+-------------+

1 row in set (0.00 sec)

3：求最大值

mysql> select max(sal) from emp;

+----------+

| max(sal) |

+----------+

|  5000.00 |

+----------+

1 row in set (0.17 sec)

4:求最小值

mysql> select min(sal) from emp;

5；统计查询(过滤掉空的记录)

 select count(ename) from emp;

+--------------+

| count(ename) |

+--------------+

|           14 |

+--------------+

1 row in set (0.00 sec)

mysql> select count(comm) from emp;

+-------------+

| count(comm) |

+-------------+

|           3 |

+-------------+

1 row in set (0.00 sec)

6:在查询结果中去重(distinct)

select job from emp;

+-----------+

| job       |

+-----------+

| CLERK     |

| SALESMAN  |

| SALESMAN  |

| MANAGER   |

| SALESMAN  |

| MANAGER   |

| MANAGERAN |

| ANALIST   |

| PRESIDENT |

| SALESMAN  |

| CLECK     |

| CLECK     |

| ANALYST   |

| CLERY     |

+-----------+

14 rows in set (0.00 sec)

mysql> select distinct job from emp;

+-----------+

| job       |

+-----------+

| CLERK     |

| SALESMAN  |

| MANAGER   |

| MANAGERAN |

| ANALIST   |

| PRESIDENT |

| CLECK     |

| ANALYST   |

| CLERY     |

+-----------+

9 rows in set (0.00 sec)

7:分组查询(group by,having) 案例:查询每个岗位的最高薪水)[先按照每个岗位分组，使用max求每组的最大值)

   select job,avg(sal) from emp  group by job having avg(sal)>1500;

+-----------+-------------+

| job       | avg(sal)    |

+-----------+-------------+

| ANALIST   | 3000.000000 |

| ANALYST   | 3000.000000 |

| MANAGER   | 2912.500000 |

| MANAGERAN | 2450.000000 |

| PRESIDENT | 5000.000000 |

+-----------+-------------+