的MySQL - 加入根據2個密鑰的表從一個鍵/值表

Question

我有設置的表，其可以被示出爲： 的object_id（INT） 鍵（VARCHAR） 值（文本）

我試圖抓住具有等於2項的鍵的object_id。

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

我知道這是行不通的，我能想到這樣做的唯一的辦法就是加入它本身：

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' 

雖然我沒有測試過的最後一條語句，我肯定這樣的事情可以工作。這個查詢將每小時執行數十萬條記錄，所以我想保持它的優化 - 有沒有更好的方法來做到這一點？

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'