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<>1、把查询结果去除重复记录【distinct】
注意：原表数据不会被修改，只是查询结果去重。 去重需要使用一个关键字：distinct mysql> select distinct job from
emp; +-----------+ | job | +-----------+ | CLERK | | SALESMAN | | MANAGER | |
ANALYST | | PRESIDENT | +-----------+ // 这样编写是错误的，语法错误。 //
distinct只能出现在所有字段的最前方。 mysql> select ename,distinct job from emp; //
distinct出现在job,deptno两个字段之前，表示两个字段联合起来去重。 mysql> select distinct job,deptno
from emp; +-----------+--------+ | job | deptno | +-----------+--------+ |
CLERK | 20 | | SALESMAN | 30 | | MANAGER | 20 | | MANAGER | 30 | | MANAGER | 10
| | ANALYST | 20 | | PRESIDENT | 10 | | CLERK | 30 | | CLERK | 10 |
+-----------+--------+ 统计一下工作岗位的数量？ select count(distinct job) from emp;
+---------------------+ | count(distinct job) | +---------------------+ | 5 |
+---------------------+