的MySQL - 加入根据2个密钥的表从一个键/值表

Question

我有设置的表，其可以被示出为： 的object_id（INT） 键（VARCHAR） 值（文本）

我试图抓住具有等于2项的键的object_id。

SELECT `object_id` FROM `settings` WHERE `key` = 'A' AND `key` = 'B' 

我知道这是行不通的，我能想到这样做的唯一的办法就是加入它本身：

SELECT a.`object_id` FROM `settings` AS a LEFT JOIN `settings` AS b ON (a.`object_id` = b.`object_id`) WHERE a.`key` = 'A' and b.`key` = 'B' 

虽然我没有测试过的最后一条语句，我肯定这样的事情可以工作。这个查询将每小时执行数十万条记录，所以我想保持它的优化 - 有没有更好的方法来做到这一点？

Answer 1

哦，你想：

SELECT `object_id` 
    FROM `settings` 
    WHERE (`key` = 'A' AND `value`='foo') 
     OR (`key`= 'B' AND `value`='bar') 
GROUP BY `object_id` 
    HAVING COUNT(*) = 2 

用途：

SELECT `object_id` 
    FROM `settings` 
    WHERE `key` IN ('A', 'B') 
GROUP BY `object_id` 
    HAVING COUNT(DISTINCT `key`) = 2 

或者：

SELECT x.`object_id` 
    FROM `settings` AS x 
    JOIN `settings` AS y ON y.`object_id` = x.`object_id` 
WHERE x.`key` = 'A' 
    AND y.`key` = 'B'