18 关于查询结果集的去重？
select distinct job from emp; // distinct 关键字 去除重复记录。
+-----------+
| job |
+-----------+
| CLERK |
| SALESMAN |
| MANAGER |
| ANALYST |
| PRESIDENT |
+-----------+
select ename,distinct job from emp;
这条语句是错误的。
记住：distinct只能出现在所有字段的最前面
select distinct deptno,job from emp;
+--------+-----------+
| deptno | job |
+--------+-----------+
| 20 | CLERK |
| 30 | SALESMAN |
| 20 | MANAGER |
| 30 | MANAGER |
| 10 | MANAGER |
| 20 | ANALYST |
| 10 | PRESIDENT |
| 30 | CLERK |
| 10 | CLERK |
+--------+-----------+
案例:统计岗位的数量?
select count(distinct job) from emp;
+---------------------+
| count(distinct job) |
+---------------------+
| 5 |
+---------------------+