在mysql查詢中選擇唯一的行

Question

我有疑問

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

而且事情是有些，我需要雇主提供獨特的記錄，但我卻重復了，因為statuss.number確實像222 6 777這樣重復，所以我只需要抓住它們一次。 我不想使用DISTINCT還有其他方法嗎？

Answer 1

使用GROUP BY語句

   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

在雇主ID上使用GROUP BY，對於返回的每一行，您只會獲得一個雇主記錄：

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，每當您在WHERE子句中引用LEFT JOINed列（如statuses.number =2 ，都將否定LEFT並強制其表現為INNER。

第二，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)