在mysql查询中选择唯一的行

Question

我有疑问

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，我需要雇主提供独特的记录，但我却重复了，因为statuss.number确实像222 6 777这样重复，所以我只需要抓住它们一次。 我不想使用DISTINCT还有其他方法吗？

Answer 1

使用GROUP BY语句

   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，对于返回的每一行，您只会获得一个雇主记录：

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，每当您在WHERE子句中引用LEFT JOINed列（如statuses.number =2 ，都将否定LEFT并强制其表现为INNER。

第二，试试这个版本：

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)