在MySQL查詢選擇唯一行

Question

我有查詢

SELECT DISTINCT employer.id, employer.name 
FROM employers 
LEFT JOIN positions ON positions.id = employers.id 
LEFT JOIN statuses ON statuses.position = positions.some 
WHERE statuses.number = 2 

而且就是一些我需要從僱主獨特的記錄，但我得到的重複，因爲statuses.number不重複像222 6 777等我需要搶他們只有一次。 我不想使用DISTINCT有沒有其他方法？

Answer 1

使用GROUP BY statment

SELECT employer.id, employer.name 
    FROM employers 
    LEFT JOIN positions ON positions.id = employers.id 
    LEFT JOIN statuses ON statuses.position = positions.some 
    WHERE statuses.number = 2 GROUP BY employer.id 

Answer 2

僱主的ID使用GROUP BY，你只會得到一個僱主記錄返回的每一行：

SELECT employer.id, employer.name 
FROM employers 
LEFT JOIN positions ON positions.id = employers.id 
LEFT JOIN statuses ON statuses.position = positions.some 
WHERE statuses.number = 2 
GROUP BY employer.id 

Answer 3

首先，任何時候你引用一個LEFT加入WHERE子句中的列，如statuses.number =2，您將否定LEFT並強制其表現得​​像INNER。

其次，試試這個版本：

SELECT e.id, e.name 
    FROM employers e 
    WHERE EXISTS(SELECT 1 
        FROM positions p 
         INNER JOIN statuses s 
          ON p.some = s.position 
        WHERE p.id = e.id 
         AND s.number = 2)