Question

我正在尝试从数据库中获取一些值，但是将其限制为每个帐户1，并且我无法按照我想要的方式运行它。

以下是查询，以正确的顺序获取电子邮件，并忽略该帐户：

SELECT ID FROM MailSent 
    WHERE DateSent IS NULL AND Valid = 1 ORDER BY Priority DESC LIMIT 14

我希望每个帐户（EmailID）具有最高优先级记录。我可以循环遍历结果并丢弃任何重复项，但实际限制将比预期的限制低很多。

以下是我尝试过的一些事情：

SELECT ID, DISTINCT(EmailID) FROM MailSent
    WHERE DateSent IS NULL AND Valid = 1 ORDER BY Priority DESC LIMIT 14
// error

SELECT DISTINCT(EmailID), ID FROM MailSent
    WHERE DateSent IS NULL AND Valid = 1 ORDER BY Priority DESC LIMIT 14
// still has duplicates

SELECT ID FROM MailSent 
    WHERE DateSent IS NULL AND Valid = 1 ORDER BY Priority DESC GROUP BY EmailID LIMIT 14
// error

SELECT ID FROM MailSent
    WHERE DateSent IS NULL AND Valid = 1 GROUP BY EmailID ORDER BY Priority DESC LIMIT 14
// wrong priority

作为奖励，但不是必需的，因为它可能很难做到，将它限制在用户定义的数量而不仅仅是1时会很好。

Answer 1

只需使用变量

SELECT ID, EmaailID
FROM (
       SELECT ID, EmailID,
              @rn := if(@email = EmailID,
                        @rn + 1,
                        if(@email := EmailID, 1, 1)
                       ) as rn
       FROM MailSent 
       CROSS JOIN (SELECT @email := 0, @rn := 0) as param
       WHERE DateSent IS NULL AND Valid = 1
       ORDER BY Priority Desc
       ) T
WHERE T.rn = 1

现在，如果您想为用户提供限制结果，则需要设置表或来自某个地方的查询。只需将以前的WHERE替换为：

   JOIN (SELECT userID, numberofRows
         FROM SettingTable) P
     ON T.ID = P.userID
    AND T.rn <= P.numberofRows

获取具有唯一值的已排序mysql记录

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