MySQL子查詢返回不正確的結果？

Question

我有以下MySQL查詢/子查詢：

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 

鑑於所列的標準，子查詢的結果（old_id）爲空（不匹配會在我的表中找到）。而不是MySQL返回null，它似乎放棄「WHERE ParentUsersValue.user_id = UsersValue.user_id」子句並選擇與其他兩個字段匹配的第一個值。這是MySQL的錯誤，還是由於某種原因的預期行爲？

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 Warning 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

這將返回正確的結果（NULL）對我來說：

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 

能否請您運行此查詢：

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

並確保它返回1？

請注意，您的子查詢不會在created上進行篩選，因此子查詢可以將值返回到主查詢定義的範圍之外。

更新：

這絕對是MySQL的錯誤。

最可能的原因是爲UsersValues選擇的訪問路徑是index_intersect。

這將從兩個索引中選擇合適的範圍並構建它們的交集。

由於該錯誤，在相交完成前評估相關子查詢，這就是爲什麼您使用正確的another_id而不是user_id得到結果。

能否請您檢查，當你用力的UsersValuesPRIMARY掃描問題仍然存在：

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 

而且，對於這個查詢，你應該對user_id和another_id創建(user_id, another_id, id)而不是兩個不同的指數的複合指數。

創建索引和重寫查詢，一點點：

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 

的user_id DESC, another_id DESC條款是邏輯冗餘，但他們將被用於順序的索引。

Answer 2

你嘗試運行子查詢只看到如果你得到正確的結果？你能告訴我們你的users_values表的模式嗎？

此外，嘗試更換您在您的子查詢SELECT id通過SELECT ParentUsersValue.id