mysql中select count(primary)、count(1)、count(*)的区别

有这样一个表结构：

 

 

mysql> desc entries;
+------------+--------------+------+-----+---------+----------------+
| Field      | Type         | Null | Key | Default | Extra          |
+------------+--------------+------+-----+---------+----------------+
| id         | int(11)      | NO   | PRI | NULL    | auto_increment |
| title       | varchar(100) | YES  |     | NULL    |                |
| content | varchar(2000)     | YES  |     | NULL    |                |
+------------+--------------+------+-----+---------+----------------+
3 rows in set (0.00 sec)

 

三种查询方式比较如下：

 

 

mysql> explain select count(id) from entries;
+----+-------------+---------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table   | type  | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+---------+-------+---------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | entries | index | NULL          | PRIMARY | 4       | NULL |   25 | Using index |
+----+-------------+---------+-------+---------------+---------+---------+------+------+-------------+
1 row in set (0.00 sec)

mysql> explain select count(1) from entries;
+----+-------------+---------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table   | type  | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+---------+-------+---------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | entries | index | NULL          | PRIMARY | 4       | NULL |   25 | Using index |
+----+-------------+---------+-------+---------------+---------+---------+------+------+-------------+
1 row in set (0.00 sec)

mysql> explain select count(*) from entries;
+----+-------------+---------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table   | type  | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+---------+-------+---------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | entries | index | NULL          | PRIMARY | 4       | NULL |   25 | Using index |
+----+-------------+---------+-------+---------------+---------+---------+------+------+-------------+
1 row in set (0.00 sec)

 

通过上面的比较，我们可以看出，这3种方法的执行过程是一模一样的，不存在性能和效率上的差异。