MySQL中count(distinct col...)组合使用的注意要点详解

mysql> select * from my_student;
+----+------------+-----------+--------+--------+
| id | number     | name      | sex    | addr   |
+----+------------+-----------+--------+--------+
|  1 | itcast0001 | Jim       | female | 北京   |
|  2 | itcast0002 | HanMeimei | female | 上海   |
|  3 | itcast0003 | Kate      | female | NULL   |
|  4 | itcast0004 | Tom       | male   | NULL   |
|  5 | itcast0005 | LinTao    | male   | NULL   |
|  6 | itcast0006 | 张越      | 女     | NULL   |
+----+------------+-----------+--------+--------+
6 rows in set (0.00 sec)

mysql> select distinct addr from my_student;
+--------+
| addr   |
+--------+
| 北京   |
| 上海   |
| NULL   |
+--------+
3 rows in set (0.00 sec)

    mysql> select count(distinct addr) from my_student;							①
	+----------------------+
	| count(distinct addr) |
	+----------------------+
	|                    2 |
	+----------------------+
	1 row in set (0.00 sec)
	
	-- 把上述SQL改写成 select count(1) from (select distinct col from ...) a; 
	mysql> select count(1) from (select distinct addr from my_student) a;		②
	+----------+
	| count(1) |
	+----------+
	|        3 |
	+----------+
	1 row in set (0.00 sec)

    mysql> select distinct sex,addr from my_student;
	+--------+--------+
	| sex    | addr   |
	+--------+--------+
	| female | 北京   |
	| female | 上海   |
	| female | NULL   |
	| male   | NULL   |
	| 女     | NULL   |
	+--------+--------+
	5 rows in set (0.00 sec)
	
	mysql> select count(distinct sex,addr) from my_student;                     ③
	+--------------------------+
	| count(distinct sex,addr) |
	+--------------------------+
	|                        2 |
	+--------------------------+
	1 row in set (0.00 sec)

	mysql> select count(1) from (select distinct sex,addr from my_student) a;	④
	+----------+
	| count(1) |
	+----------+
	|        5 |
	+----------+
	1 row in set (0.00 sec)