Mysql的GROUP BY和ORDER BY DESC

Question

這是我的全部查詢：

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 

有一個與第三聯接導致腳本執行約1分鐘的一個問題。當我在PMA中分開運行它時：

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

它仍然會很長時間才能執行。

我想要的是得到一個，最後添加從contacts排爲每一個客戶是誰在組4（客戶端可以在不同的羣體）。

謝謝。

Answer 1

得到「最後從每個客戶端誰是第4組中的聯繫人添加的行」試試這個：

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中contacts則不需要第二個連接子句。

默認情況下，MySQL的所有排序GROUP BY COL1，COL2，...查詢，如果你 指定ORDER BY COL1，COL2，......在查詢也是如此。如果您 包括ORDER BY子句中明確包含在同一列 列表中，MySQL優化它拿走，沒有任何速度的損失，雖然 排序仍然存在。如果查詢包括GROUP BY但你要 避免排序結果的開銷，可以抑制由 排序指定ORDER BY NULL。

完整docs。

全面查詢：

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