MySQL语句，in子查询语法错误，却不影响整个查询的正确性

问题重现

比如我有两个表

mysql> select * from user;
+----+------+-----+
| id | name | age |
+----+------+-----+
|  1 | A    |   9 |
|  2 | B    |  11 |
|  3 | C    |  15 |
|  4 | D    |  13 |
+----+------+-----+
4 rows in set (0.03 sec)

mysql> select * from blacklist;
+----+------+------+
| id | name | type |
+----+------+------+
|  1 | B    |    1 |
|  2 | C    |    0 |
|  3 | D    |    1 |
+----+------+------+
3 rows in set (0.03 sec)

我要从里面选取黑名单并且年龄大于10的。可是我手误，把age条件加在了in查询里，可是黑名单没有age字段。

select * from user where name in (select name from blacklist where type = 1 and age > 10);

单独执行子查询是有很明显的问题的：

mysql> select name from blacklist where type = 1 and age > 10;
1054 - Unknown column 'age' in 'where clause'

可是我如果执行整个查询：

mysql> select * from user where name in (select name from blacklist where type = 1 and age > 10);
+----+------+-----+
| id | name | age |
+----+------+-----+
|  2 | B    |  11 |
|  4 | D    |  13 |
+----+------+-----+
2 rows in set (0.02 sec)

没问题！并且还是正确的结果！！

为什么？

我们explain一下

mysql> explain select * from user where name in (select name from blacklist where type = 1 and age > 10);
+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+-------------------------------------------------------------------+
| id | select_type | table     | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra                                                             |
+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+-------------------------------------------------------------------+
|  1 | SIMPLE      | blacklist | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |    33.33 | Using where; Start temporary                                      |
|  1 | SIMPLE      | user      | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    4 |    25.00 | Using where; End temporary; Using join buffer (Block Nested Loop) |
+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+-------------------------------------------------------------------+
2 rows in set (0.02 sec)

没看出什么端倪，继续执行

mysql> show warnings;
+-------+------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Level | Code | Message                                                                                                                                                                                                                                                                                                        |
+-------+------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Note  | 1276 | Field or reference 'mytest.user.age' of SELECT #2 was resolved in SELECT #1                                                                                                                                                                                                                                    |
| Note  | 1003 | /* select#1 */ select `mytest`.`user`.`id` AS `id`,`mytest`.`user`.`name` AS `name`,`mytest`.`user`.`age` AS `age` from `mytest`.`user` semi join (`mytest`.`blacklist`) where ((`mytest`.`user`.`name` = `mytest`.`blacklist`.`name`) and (`mytest`.`blacklist`.`type` = 1) and (`mytest`.`user`.`age` > 10)) |
+-------+------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
2 rows in set (0.03 sec)

这里能看见数据库优化之后的语句

/* select#1 */ 
SELECT
`mytest`.`user`.`id` AS `id`,
`mytest`.`user`.`name` AS `name`,
`mytest`.`user`.`age` AS `age` 
FROM
    `mytest`.`user` semi
    JOIN ( `mytest`.`blacklist` ) 
WHERE
(
    (`mytest`.`user`.`name` = `mytest`.`blacklist`.`name` ) 
    AND ( `mytest`.`blacklist`.`type` = 1 ) 
    AND ( `mytest`.`user`.`age` > 10 )
)

可以看出，数据库把原来的查询改成了join查询，每个字段都指向了正确的表。

解决

当然是把语句改正呗，这属于手误造成的。

mysql> select * from user where name in (select name from blacklist where type = 1) and age > 10;
+----+------+-----+
| id | name | age |
+----+------+-----+
|  2 | B    |  11 |
|  4 | D    |  13 |
+----+------+-----+
2 rows in set (0.03 sec)

但是其次，我们看到in里面如果有语法错误，是没有暴露的。我们把in改成exists试一下：

mysql> select * from user where EXISTS (select name from blacklist where type = 1 and age > 10);
+----+------+-----+
| id | name | age |
+----+------+-----+
|  2 | B    |  11 |
|  3 | C    |  15 |
|  4 | D    |  13 |
+----+------+-----+
3 rows in set (0.05 sec)

结果不对，并且EXISTS返回的是一个布尔值，只要其中的子查询返回行数，where条件即成立。所以，select name 换成select 1也是成立的。

mysql> select * from user where EXISTS (select 1 from blacklist where type = 1 and age > 10);
+----+------+-----+
| id | name | age |
+----+------+-----+
|  2 | B    |  11 |
|  3 | C    |  15 |
|  4 | D    |  13 |
+----+------+-----+
3 rows in set (0.04 sec)

如果使EXISTS不成立，则

mysql> select * from user where EXISTS (select name from blacklist where type = 11 and age > 10);
Empty set

explain一下

mysql> explain select * from user where EXISTS (select name from blacklist where type = 1 and age > 10);
+----+--------------------+-----------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type        | table     | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+--------------------+-----------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | PRIMARY            | user      | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    4 |   100.00 | Using where |
|  2 | DEPENDENT SUBQUERY | blacklist | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |    33.33 | Using where |
+----+--------------------+-----------+------------+------+---------------+------+---------+------+------+----------+-------------+
2 rows in set (0.06 sec)

查看warnings

mysql> show warnings;
+-------+------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Level | Code | Message                                                                                                                                                                                                                                                                                                      |
+-------+------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Note  | 1276 | Field or reference 'mytest.user.age' of SELECT #2 was resolved in SELECT #1                                                                                                                                                                                                                                  |
| Note  | 1003 | /* select#1 */ select `mytest`.`user`.`id` AS `id`,`mytest`.`user`.`name` AS `name`,`mytest`.`user`.`age` AS `age` from `mytest`.`user` where exists(/* select#2 */ select `mytest`.`blacklist`.`name` from `mytest`.`blacklist` where ((`mytest`.`blacklist`.`type` = 1) and (`mytest`.`user`.`age` > 10))) |
+-------+------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
2 rows in set (0.06 sec)

优化之后的语句

/* select#1 */ 
SELECT
`mytest`.`user`.`id` AS `id`,
`mytest`.`user`.`name` AS `name`,
`mytest`.`user`.`age` AS `age` 
FROM
    `mytest`.`user` 
WHERE
    EXISTS (
/* select#2 */
    SELECT
        `mytest`.`blacklist`.`name` 
    FROM
        `mytest`.`blacklist` 
    WHERE
        ((
                `mytest`.`blacklist`.`type` = 1 
            ) 
    AND ( `mytest`.`user`.`age` > 10 )))

可以看到，使用了exists，同样优化掉了低级错误。如果想要得到我们预期的数据，需要在exists子查询添加一个条件

mysql> select * from user a where EXISTS (select 1 from blacklist where type = 1 and name = a.name and age > 10);
+----+------+-----+
| id | name | age |
+----+------+-----+
|  2 | B    |  11 |
|  4 | D    |  13 |
+----+------+-----+
2 rows in set (0.06 sec)

exists可以看做一个循环

List<User> result = new ArrayList<>();
List<User> user = new ArrayList<>();
for (int i = 0; i < user.size(); i++){
    if (exists(black.type == 1 && black.name.equals(user.get(i).name)) && user.get(i).age > 10){
        result.add(user.get(i));
    }
}

不是很严谨，但是大概是这么个意思

IN和Exists的区别

Exists：先执行外部查询语句，然后在执行子查询，子查询中它每次都会去执行数据库的查询，执行次数等于外查询的数据数量。

In：先查询 in()子查询的数据(1次)，并且将数据放进内存里(不需要多次查询)，然后外部查询的表再根据查询的结果进行查询过滤，最后返回结果。

In 是把外表和内表作hash 连接，而exists是对外表作loop循环，每次loop循环再对内表进行查询。

参考：https://blog.csdn.net/qq_27409289/article/details/85963089