sql - MySQL 子查询返回不正确的结果？

Question

我有以下 MySQL 查询/子查询:

SELECT id, user_id, another_id, myvalue, created, modified,
(
    SELECT id 
    FROM users_values AS ParentUsersValue
    WHERE ParentUsersValue.user_id = UsersValue.user_id
    AND ParentUsersValue.another_id = UsersValue.another_id 
    AND ParentUsersValue.id < UsersValue.id 
    ORDER BY id DESC 
    LIMIT 1
) AS old_id

FROM users_values AS UsersValue
WHERE created >= '2009-12-20' 
AND created <= '2010-01-21' 
AND user_id = 9917
AND another_id = 23

鉴于列出的条件，子查询 (old_id) 的结果应该为空(在我的表中找不到匹配项)。 MySQL 没有返回 null，它似乎只是删除了“WHERE ParentUsersValue.user_id = UsersValue.user_id”子句并选择与其他两个字段匹配的第一个值。这是一个 MySQL 错误，还是由于某种原因这是预期的行为？

更新:

CREATE TABLE users_values (
    id int(11) NOT NULL AUTO_INCREMENT,
    user_id int(11) DEFAULT NULL,
    another_id int(11) DEFAULT NULL,
    myvalue double DEFAULT NULL,
    created datetime DEFAULT NULL,
    modified datetime DEFAULT NULL,
    PRIMARY KEY (id)
) ENGINE=InnoDB AUTO_INCREMENT=2801 DEFAULT CHARSET=latin1

EXPLAIN EXTENDED:

id  select_type table   type    possible_keys   key key_len ref rows    filtered    Extra
1   PRIMARY UsersValue  index_merge user_id,another_id  user_id,another_id  5,5 NULL    1   100.00  Using intersect(user_id,another_id); Using where
2   DEPENDENT SUBQUERY  ParentUsersValue    index   PRIMARY,user_id,another_id  PRIMARY 4   NULL    1   100.00  Using where

EXPLAIN EXTENDED 警告 1003:

select `mydb`.`UsersValue`.`id` AS `id`,`mydb`.`UsersValue`.`user_id` AS `user_id`,`mydb`.`UsersValue`.`another_id` AS `another_id`,`mydb`.`UsersValue`.`myvalue` AS `myvalue`,`mydb`.`UsersValue`.`created` AS `created`,`mydb`.`UsersValue`.`modified` AS `modified`,(select `mydb`.`ParentUsersValue`.`id` AS `id` from `mydb`.`users_values` `ParentUsersValue` where ((`mydb`.`ParentUsersValue`.`user_id` = `mydb`.`UsersValue`.`user_id`) and (`mydb`.`ParentUsersValue`.`another_id` = `mydb`.`UsersValue`.`another_id`) and (`mydb`.`ParentUsersValue`.`id` < `mydb`.`UsersValue`.`id`)) order by `mydb`.`ParentUsersValue`.`id` desc limit 1) AS `old_id` from `mydb`.`users_values` `UsersValue` where ((`mydb`.`UsersValue`.`another_id` = 23) and (`mydb`.`UsersValue`.`user_id` = 9917) and (`mydb`.`UsersValue`.`created` >= '2009-12-20') and (`mydb`.`UsersValue`.`created` <= '2010-01-21'))

Answer 1

这会为我返回正确的结果(NULL):

CREATE TABLE users_values (id INT NOT NULL PRIMARY KEY, user_id INT NOT NULL, another_id INT NOT NULL, created DATETIME NOT NULL);

INSERT
INTO    users_values VALUES (1, 9917, 23, '2010-01-01');

SELECT  *,
        (
        SELECT  id
        FROM    users_values AS ParentUsersValue
        WHERE   ParentUsersValue.user_id = UsersValue.user_id
                AND ParentUsersValue.another_id = UsersValue.another_id
                AND ParentUsersValue.id < UsersValue.id
        ORDER BY id
                DESC
        LIMIT 1
        ) AS old_id
FROM    users_values AS UsersValue
WHERE   created >= '2009-12-20'
        AND created <= '2010-01-21'
        AND user_id = 9917
        AND another_id = 23

你能运行这个查询吗:

SELECT  COUNT(*)
FROM    users_values AS UsersValue
WHERE   user_id = 9917
        AND another_id = 23

并确保它返回 1 ？

请注意，您的子查询不会过滤 created , 因此子查询可以返回主查询定义范围之外的值。

更新:

这绝对是 MySQL 中的错误.

最有可能的原因是为 UsersValues 选择的访问路径是index_intersect .

这会从两个索引中选择适当的范围并构建它们的交集。

由于这个错误，依赖子查询在交集完成之前被评估，这就是为什么你得到的结果是正确的 another_id但错了user_id .

当你强制PRIMARY时，你能检查问题是否仍然存在吗？扫描 UsersValues :

SELECT  *,
        (
        SELECT  id
        FROM    users_values AS ParentUsersValue
        WHERE   ParentUsersValue.user_id = UsersValue.user_id
                AND ParentUsersValue.another_id = UsersValue.another_id
                AND ParentUsersValue.id < UsersValue.id
        ORDER BY id
                DESC
        LIMIT 1
        ) AS old_id
FROM    users_values AS UsersValue FORCE INDEX (PRIMARY)
WHERE   created >= '2009-12-20'
        AND created <= '2010-01-21'
        AND user_id = 9917
        AND another_id = 23

此外，对于此查询，您应该在 (user_id, another_id, id) 上创建一个复合索引而不是 user_id 上的两个不同索引和 another_id .

创建索引并稍微重写查询:

SELECT  *,
        (
        SELECT  id
        FROM    users_values AS ParentUsersValue
        WHERE   ParentUsersValue.user_id = UsersValue.user_id
                AND ParentUsersValue.another_id = UsersValue.another_id
                AND ParentUsersValue.id < UsersValue.id
        ORDER BY
                user_id DESC, another_id DESC, id DESC
        LIMIT 1
        ) AS old_id
FROM    users_values AS UsersValue
WHERE   created >= '2009-12-20'
        AND created <= '2010-01-21'
        AND user_id = 9917
        AND another_id = 23

user_id DESC, another_id DESC子句在逻辑上是多余的，但它们将使索引用于排序。