今天碰到一個關于exists很奇怪的問題
第一個語句如下：
SELECTcount(1)FROMAPPLY tWHEREEXISTS (SELECTr.APPLY_IDFROMRECORD rWHEREt.APPLY_ID = r.APPLY_ID);
產生的結果是：89584
第二個語句如下：
SELECTcount(1)FROMAPPLY tWHEREEXISTS (SELECTmax(r.FINISH_TIME)FROMRECORD rWHEREt.APPLY_ID = r.APPLY_ID);
產生的結果是：432382
確實相當奇怪，對于exist子句來說，其判斷的是子查詢的值是否存在，也就是說，列名，和對列名求最大值沒什么區別啊。
包括MySQL官方文檔中也提到
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會自動忽略到SELECT的列表。
后來在自己的環境測試了一下，確實是MySQL的一個bug
測試環境：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 |
很明顯，id等于2的列不存在，但是第二條語句還是當做TRUE來處理了。
也確認了下兩條語句的執行計劃和改寫后的SQL
第一個語句
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 |+---------+------+-------------------------------------------------------------------+
第二個語句
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)
執行計劃及改寫后的SQL確實有所不同，看來，確實是MySQL的一個bug了。
于是，給官方提了個bug
http://bugs.mysql.com/bug.php?id=82562
總結
建議寫exists語句時，子查詢中直接用*，而不用對列進行任何函數操作，避免碰到官方bug，
事實上，對于abs，floor函數又沒問題
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)
以上所述是小編給大家介紹的MySQL關于exists的一個bug ，希望對大家有所幫助，如果大家有任何疑問請給我留言，小編會及時回復大家的。在此也非常感謝大家對VeVb武林網網站的支持！
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