count(1)、count(*)与count(列名)的执行区别详解

mysql> create table counttest(name char(1), age char(2));
Query OK, 0 rows affected (0.03 sec)

mysql> insert into counttest values
  -> ('a', '14'),('a', '15'), ('a', '15'), 
  -> ('b', NULL), ('b', '16'), 
  -> ('c', '17'),
  -> ('d', null), 
  ->('e', '');
Query OK, 8 rows affected (0.01 sec)
Records: 8 Duplicates: 0 Warnings: 0

mysql> select  from counttest;
+------+------+
| name | age |
+------+------+
| a  | 14  |
| a  | 15  |
| a  | 15  |
| b  | NULL |
| b  | 16  |
| c  | 17  |
| d  | NULL |
| e  |   |
+------+------+
8 rows in set (0.00 sec)

mysql> select name, count(name), count(1), count(), count(age), count(distinct(age))
  -> from counttest
  -> group by name;
+------+-------------+----------+----------+------------+----------------------+
| name | count(name) | count(1) | count() | count(age) | count(distinct(age)) |
+------+-------------+----------+----------+------------+----------------------+
| a  |      3 |    3 |    3 |     3 |          2 |
| b  |      2 |    2 |    2 |     1 |          1 |
| c  |      1 |    1 |    1 |     1 |          1 |
| d  |      1 |    1 |    1 |     0 |          0 |
| e  |      1 |    1 |    1 |     1 |          1 |
+------+-------------+----------+----------+------------+----------------------+
5 rows in set (0.00 sec)