《Mysql应用MySQL关于exists的一个bug》要点：
本文介绍了Mysql应用MySQL关于exists的一个bug，希望对您有用。如果有疑问，可以联系我们。
MYSQL必读今天碰到一个关于exists很奇怪的问题
MYSQL必读第一个语句如下：
MYSQL必读
SELECT
count(1)
FROM
APPLY t
WHERE
EXISTS (
SELECT
r.APPLY_ID
FROM
RECORD r
WHERE
t.APPLY_ID = r.APPLY_ID
);
MYSQL必读产生的结果是：89584
MYSQL必读第二个语句如下：
MYSQL必读
SELECT
count(1)
FROM
APPLY t
WHERE
EXISTS (
SELECT
max(r.FINISH_TIME)
FROM
RECORD r
WHERE
t.APPLY_ID = r.APPLY_ID
);
MYSQL必读产生的结果是：432382
MYSQL必读确实相当奇怪,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.
MYSQL必读包括MySQL官方文档中也提到
MYSQL必读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.
MYSQL必读大意就是MySQL会自动忽略到SELECT的列表.
MYSQL必读后来在自己的环境测试了一下,确实是MySQL的一个bug
MYSQL必读测试环境：MySQL 5.6.31,5.7.14
MYSQL必读
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 |
MYSQL必读很明显,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.
MYSQL必读也确认了下两条语句的执行计划和改写后的SQL
MYSQL必读第一个语句
MYSQL必读
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 |
+---------+------+-------------------------------------------------------------------+
MYSQL必读第二个语句
MYSQL必读
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)
MYSQL必读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.
MYSQL必读于是,给官方提了个bug
MYSQL必读http://bugs.mysql.com/bug.php?id=82562
MYSQL必读总结
MYSQL必读建议写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,
MYSQL必读事实上,对于abs,floor函数又没问题
MYSQL必读
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)
MYSQL必读以上所述是小编给大家介绍的MySQL关于exists的一个bug ,希望对大家有所赞助,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维易PHP网站的支持!
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