在我的研究中，我可以发现结果越来越快。

select * from table where condition=value
(1 total, Query took 0.0052 sec)

select exists(select * from table where condition=value)
(1 total, Query took 0.0008 sec)

select count(*) from table where condition=value limit 1) 
(1 total, Query took 0.0007 sec)

select exists(select * from table where condition=value limit 1)
(1 total, Query took 0.0006 sec) 

下面是@ChrisThompson回答的一个小例子

例子:

mysql> SELECT * FROM table_1;
+----+--------+
| id | col1   |
+----+--------+
|  1 | foo    |
|  2 | bar    |
|  3 | foobar |
+----+--------+
3 rows in set (0.00 sec)

mysql> SELECT EXISTS(SELECT 1 FROM table_1 WHERE id = 1);
+--------------------------------------------+
| EXISTS(SELECT 1 FROM table_1 WHERE id = 1) |
+--------------------------------------------+
|                                          1 |
+--------------------------------------------+
1 row in set (0.00 sec)

mysql> SELECT EXISTS(SELECT 1 FROM table_1 WHERE id = 9);
+--------------------------------------------+
| EXISTS(SELECT 1 FROM table_1 WHERE id = 9) |
+--------------------------------------------+
|                                          0 |
+--------------------------------------------+
1 row in set (0.00 sec)

使用别名:

mysql> SELECT EXISTS(SELECT 1 FROM table_1 WHERE id = 1) AS mycheck;
+---------+
| mycheck |
+---------+
|       1 |
+---------+
1 row in set (0.00 sec)

你也可以试试EXISTS:

SELECT EXISTS(SELECT * FROM table1 WHERE ...)

根据文档，你可以选择任何东西。

传统上，EXISTS子查询以SELECT *开头，但也可以这样 以SELECT 5或SELECT column n1或任何东西开始。MySQL 在这样的子查询中忽略SELECT列表，所以没有区别。

COUNT查询更快，虽然可能不是很明显，但就获得所需的结果而言，两者都应该足够了。

在我的研究中，我可以发现结果越来越快。

select * from table where condition=value
(1 total, Query took 0.0052 sec)

select exists(select * from table where condition=value)
(1 total, Query took 0.0008 sec)

select count(*) from table where condition=value limit 1) 
(1 total, Query took 0.0007 sec)

select exists(select * from table where condition=value limit 1)
(1 total, Query took 0.0006 sec) 

建议你不要使用Count，因为Count总是为db做额外的负载，使用SELECT 1，如果你的记录在那里，它返回1，否则它返回null，你可以处理它。