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今天碰到一个很奇怪的问题，关于 exists 的，
第一个语句如下：
| SELECT | |
| count(1) | |
| FROM | |
| APPLY t | |
| WHERE | |
| EXISTS (SELECT | |
| r.APPLY_ID | |
| FROM | |
| RECORD r | |
| WHERE | |
| t.APPLY_ID = r.APPLY_ID | |
| ); |
产生的结果是：89584
第二个语句如下：
| SELECT | |
| count(1) | |
| FROM | |
| APPLY t | |
| WHERE | |
| EXISTS (SELECT | |
| max(r.FINISH_TIME) | |
| FROM | |
| RECORD r | |
| WHERE | |
| t.APPLY_ID = r.APPLY_ID | |
| ); |
产生的结果是：432382
确实相当奇怪，对于 exist 子句来说，其判断的是子查询的值是否存在，也就是说，列名，和对列名求最大值没什么区别啊。
包括 MySQL 官方文档中也提到
Traditionally, an EXISTS subquery starts with SELECT *, but it could begin with SELECT 5 or SELECT column1 or anything at all. MySQL ignores the SELECT list in such a subquery, so it makes no difference.
大意就是 MySQL 会自动忽略到 SELECT 的列表。
后来在自己的环境测试了一下，确实是 MySQL 的一个 bug
测试环境：MySQL 5.6.31，5.7.14
| mysql> create table t3(id int,t datetime); | |
| Query OK, 0 rows affected (0.44 sec) | |
| mysql> insert into t3 values(1,'20160812'); | |
| Query OK, 1 row affected (0.16 sec) | |
| mysql> select 1 from dual where exists (select id from t3 where id=2); | |
| Empty set (0.15 sec) | |
| mysql> select 1 from dual where exists (select max(id) from t3 where id=2); | |
| +---+ | |
| | 1 | | |
| +---+ | |
| | 1 | |
很明显，id 等于 2 的列不存在，但是第二条语句还是当做 TRUE 来处理了。
也确认了下两条语句的执行计划和改写后的 SQL
第一个语句
| mysql> EXPLAIN EXTENDED select 1 from dual where exists (select id from t3 where id=2); | |
| +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------+ | |
| | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | | |
| +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------+ | |
| | 1 | PRIMARY | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Impossible WHERE | | |
| | 2 | SUBQUERY | t3 | NULL | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | Using where | | |
| +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------+ | |
| 2 rows in set, 2 warnings (0.00 sec) | |
| mysql> show warnings; | |
| +---------+------+-------------------------------------------------------------------+ | |
| | Level | Code | Message | | |
| +---------+------+-------------------------------------------------------------------+ | |
| | Warning | 1681 | 'EXTENDED' is deprecated and will be removed in a future release. | | |
| | Note | 1003 | /* select#1 */ select 1 AS `1` from DUAL where 0 | | |
| +---------+------+-------------------------------------------------------------------+ |
第二个语句
| mysql> EXPLAIN EXTENDED select 1 from dual where exists (select max(id) from t3 where id=2); | |
| +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+ | |
| | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | | |
| +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+ | |
| | 1 | PRIMARY | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | No tables used | | |
| | 2 | SUBQUERY | t3 | NULL | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | Using where | | |
| +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+ | |
| 2 rows in set, 2 warnings (0.00 sec) | |
| mysql> show warnings; | |
| +---------+------+-------------------------------------------------------------------+ | |
| | Level | Code | Message | | |
| +---------+------+-------------------------------------------------------------------+ | |
| | Warning | 1681 | 'EXTENDED' is deprecated and will be removed in a future release. | | |
| | Note | 1003 | /* select#1 */ select 1 AS `1` from DUAL where 1 | | |
| +---------+------+-------------------------------------------------------------------+ | |
| 2 rows in set (0.00 sec) |
执行计划及改写后的 SQL 确实有所不同，看来，确实是 MySQL 的一个 bug 了。
于是，给官方提了个 bug
http://bugs.mysql.com/bug.php?id=82562
总结
建议写 exists 语句时，子查询中直接用 *，而不用对列进行任何函数操作，避免碰到官方 bug，
事实上，对于 abs，floor 函数又没问题
| mysql> select 1 from dual where exists (select abs(id) from t3 where id=2); | |
| Empty set (0.07 sec) | |
| mysql> select 1 from dual where exists (select floor(id) from t3 where id=2); | |
| Empty set (0.00 sec) |
后续
这个确实不是一个 bug，这个是符合 SQL 标准的。
可以参考官方的回复
http://bugs.mysql.com/bug.php?id=82562
本文永久更新链接地址 ：http://www.linuxidc.com/Linux/2016-08/134344.htm
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