7

这是我的完整查询：

SELECT * FROM `clientgroupassign`
LEFT JOIN `clients` ON `clientgroupassign`.clientId = `clients`.clientId
LEFT JOIN `users` ON `carerId` = `userId`
LEFT JOIN 
    (SELECT * FROM 
        (SELECT * FROM `contacts` WHERE `contactGroup` = 4 ORDER BY `contactId` DESC) 
        as `contacts` 
    GROUP BY (`contactClientId`)
    )  AS `contacts` ON `contactClientId` = `clients`.clientId
WHERE groupId = 4
ORDER BY `clients`.clientId

第三次连接存在问题，导致脚本执行大约 1 分钟。当我在 PMA 中单独运行它时：

SELECT * FROM (SELECT * FROM `contacts` WHERE `contactGroup` = 4 ORDER BY `contactId` DESC) AS `contacts` GROUP BY (`contactClientId`)

执行起来仍然很长。

我想要的是为contacts组 4 中的每个客户获取一个最后添加的行（客户可以在不同的组中）。

谢谢。

4

1 回答 1

6

要获取“组 4 中每个客户的联系人的最后添加行”，请尝试以下操作：

SELECT
    c.*
FROM(
    SELECT
        contactClientId,
        MAX(contactId) as cid
    FROM
        contacts
    WHERE
        contactGroup = 4
    GROUP BY
        contactClientId
    ORDER BY
        NULL
) as tmp
INNER JOIN contacts as c
    ON c.contactId = tmp.cid
    AND c.contactClientId = tmp.contactClientId

如果contactId是 PK，contacts则不需要第二个连接子句。

默认情况下，MySQL 对所有 GROUP BY col1, col2, ... 查询进行排序，就好像您在查询中也指定了 ORDER BY col1, col2, ... 一样。如果你显式地包含一个包含相同列列表的 ORDER BY 子句，MySQL 会优化它而不会造成任何速度损失，尽管排序仍然会发生。如果查询包含 GROUP BY 但您想避免对结果进行排序的开销，则可以通过指定 ORDER BY NULL 来抑制排序。

完整的文档。

完整查询：

SELECT * FROM `clientgroupassign`
LEFT JOIN `clients` ON `clientgroupassign`.clientId = `clients`.clientId
LEFT JOIN `users` ON `carerId` = `userId`
LEFT JOIN 
    (
        SELECT
            c.*
        FROM(
            SELECT
                contactClientId,
                MAX(contactId) as cid
            FROM
                contacts
            WHERE
                contactGroup = 4
            GROUP BY
                contactClientId
            ORDER BY
                NULL
        ) as tmp
        INNER JOIN contacts as c
            ON c.contactId = tmp.cid
            AND c.contactClientId = tmp.contactClientId 
    )  AS `contacts` ON `contactClientId` = `clients`.clientId
WHERE groupId = 4
ORDER BY `clients`.clientId
于 2013-09-16T15:08:19.550 回答