day02-1 distinct关键字

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 |
	+---------------------+