SELECT COUT(主键)、count(1)、count(*)的区别

[你们都是坏人]

SELECT COUT(主键)、count(1)、count(*)的区别

[fcoolx]

根据explian 得出结论 没区别

mysql> explain select count(1) from table1;
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
| 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Select tables optimized away |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
1 row in set (0.00 sec)

mysql> explain select count(*) from table1;
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
| 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Select tables optimized away |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
1 row in set (0.00 sec)

mysql> explain select count(id) from table1;
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
| 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Select tables optimized away |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+

[随风醉舞]

返回table表里面的记录总数
select count(1) from table的效率更好。

[懒得去死]

效果是一样的，性能也没有啥区别！

[zhifeiji007]

select cout(主键),好像没有见过。
其他两个能得出相同结果。

[wwwwb]

不是吧，COUNT（*）count(1)是算NULL行的，其它不算NULL

[wwwwb]

如果只是主键，那么3个SQL语句结果是一致的，因为主键不能为NULL，如是其它字段，且有NULL的话，结果是
有区别的

[WWWWA]

增加一条有NULL的记录测试一下就知道

[xiang3630]

增加一条有NULL的记录测试一下就知道

测了，都一样！50000条数据