MySQL去重distinct

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 <字段名1>[,<字段名2>...,<字段名n>] from <表名>;

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 <字段名1>[,<字段名2>...,<字段名n>]) from <表名>;

示例：

mysql> select count(distinct deptno,job) from emp;
+----------------------------+
| count(distinct deptno,job) |
+----------------------------+
|                          9 |
+----------------------------+