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：distinctmysql> 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 |+---------------------+
