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本文介紹了Mysql應(yīng)用MySQL關(guān)于exists的一個(gè)bug，希望對您有用。如果有疑問，可以聯(lián)系我們。
MYSQL必讀今天碰到一個(gè)關(guān)于exists很奇怪的問題
MYSQL必讀第一個(gè)語句如下：
MYSQL必讀
SELECT
count(1)
FROM
APPLY t
WHERE
EXISTS (
SELECT
r.APPLY_ID
FROM
RECORD r
WHERE
t.APPLY_ID = r.APPLY_ID
);
MYSQL必讀產(chǎn)生的結(jié)果是：89584
MYSQL必讀第二個(gè)語句如下：
MYSQL必讀
SELECT
count(1)
FROM
APPLY t
WHERE
EXISTS (
SELECT
max(r.FINISH_TIME)
FROM
RECORD r
WHERE
t.APPLY_ID = r.APPLY_ID
);
MYSQL必讀產(chǎn)生的結(jié)果是：432382
MYSQL必讀確實(shí)相當(dāng)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(qū)別啊.
MYSQL必讀包括MySQL官方文檔中也提到
MYSQL必讀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必讀大意就是MySQL會自動忽略到SELECT的列表.
MYSQL必讀后來在自己的環(huán)境測試了一下,確實(shí)是MySQL的一個(gè)bug
MYSQL必讀測試環(huán)境：MySQL 5.6.31,5.7.14
MYSQL必讀
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必讀很明顯,id等于2的列不存在,但是第二條語句還是當(dāng)做TRUE來處理了.
MYSQL必讀也確認(rèn)了下兩條語句的執(zhí)行計(jì)劃和改寫后的SQL
MYSQL必讀第一個(gè)語句
MYSQL必讀
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必讀第二個(gè)語句
MYSQL必讀
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必讀執(zhí)行計(jì)劃及改寫后的SQL確實(shí)有所不同,看來,確實(shí)是MySQL的一個(gè)bug了.
MYSQL必讀于是,給官方提了個(gè)bug
MYSQL必讀http://bugs.mysql.com/bug.php?id=82562
MYSQL必讀總結(jié)
MYSQL必讀建議寫exists語句時(shí),子查詢中直接用*,而不用對列進(jìn)行任何函數(shù)操作,避免碰到官方bug,
MYSQL必讀事實(shí)上,對于abs,floor函數(shù)又沒問題
MYSQL必讀
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必讀以上所述是小編給大家介紹的MySQL關(guān)于exists的一個(gè)bug ,希望對大家有所贊助,如果大家有任何疑問請給我留言,小編會及時(shí)回復(fù)大家的.在此也非常感謝大家對維易PHP網(wǎng)站的支持!
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