MySQL关于exists的一个bug

今天碰到一个很奇怪的问题，关于exists的，第一个语句如下：SELECTcount（1）FROMAPPLY tWHEREEXISTS （SELECTr.APPLY_IDFROMRECORD rWHEREt.APPLY_ID = r.APPLY_ID）;产生的结果是：89584第二个语句如下：SELECTcount（1）FROMAPPLY tWHEREEXISTS （SELECTmax（r.FINISH_TIME）FROMRECORD rWHEREt.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 whereexists （select id from t3 where id=2）;Empty set （0.15 sec）mysql> select 1 from dual whereexists （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 whereexists （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 DUALwhere 0 |+---------+------+-------------------------------------------------------------------+ 第二个语句mysql> EXPLAIN EXTENDED select 1 from dual whereexists （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 DUALwhere 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bughttp://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 whereexists （select abs（id） from t3 where id=2）;Empty set （0.07 sec）mysql> select 1 from dual whereexists （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