去重

在MySQL中需要查询表中不重复的记录时，可以使用distinct关键字过滤重复记录。

语法：

select distinct [,...,] from ;

数据表如下：

mysql> select * from emp;

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

| empno | ename | job | mgr | hiredate | sal | comm | deptno |

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

| 7369 | smith | clerk | 7902 | 1980-12-17 | 800 | NULL | 20 |

| 7499 | allen | salesman | 7698 | 1981-02-20 | 1600 | 300 | 30 |

| 7521 | ward | salesman | 7698 | 1981-02-22 | 1250 | 500 | 30 |

| 7566 | jones | manager | 7839 | 1981-04-02 | 2975 | NULL | 20 |

| 7654 | martin | salesman | 7698 | 1981-09-28 | 1250 | 1400 | 30 |

| 7698 | blake | manager | 7839 | 1981-05-01 | 2850 | NULL | 30 |

| 7782 | clark | manager | 7839 | 1981-06-09 | 2450 | NULL | 10 |

| 7788 | scott | analyst | 7566 | 1987-04-19 | 3000 | NULL | 20 |

| 7839 | king | persident | NULL | 1981-11-17 | 5000 | NULL | 10 |

| 7844 | turner | salesman | 7698 | 1981-09-08 | 1500 | 0 | 30 |

| 7876 | adams | clerk | 7788 | 1987-05-23 | 1100 | NULL | 20 |

| 7900 | james | clerk | 7698 | 1981-12-03 | 950 | NULL | 30 |

| 7902 | ford | analyst | 7566 | 1981-12-03 | 3000 | NULL | 20 |

| 7934 | miller | clerk | 7782 | 1982-01-23 | 1300 | NULL | 10 |

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

示例1：单个字段去重

mysql> select distinct deptno from emp;

+--------+

| deptno |

+--------+

| 20 |

| 30 |

| 10 |

+--------+

mysql> select distinct job from emp;

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

| job |

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

| clerk |

| salesman |

| manager |

| analyst |

| persident |

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

示例2：多个字段去重

mysql> select distinct deptno,job from emp;

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

| deptno | job |

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

| 20 | clerk |

| 30 | salesman |

| 20 | manager |

| 30 | manager |

| 10 | manager |

| 20 | analyst |

| 10 | persident |

| 30 | clerk |

| 10 | clerk |

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

多个字段去重时，distinct关键字必须位于第一个字段前，多个字段完全一样的情况下，才会过滤。

非重复计数：

select count(distinct [,...,]) from ;

示例：

mysql> select count(distinct deptno,job) from emp;

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

| count(distinct deptno,job) |

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

| 9 |

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