今天碰到一个关于exists很奇怪的问题
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)
以上所述是小编给大家介绍的MySQL关于exists的一个bug ，希望对大家有所帮助，如果大家有任何疑问请给我留言，小编会及时回复大家的。在此也非常感谢大家对脚本之家网站的支持！
