MySQL：多表連接

Question

我有一個系統可以請求驅動程序。 如果不止一位司機請求同一行程，它會轉到首先與我們一起工作的司機。如果他們在同一天開始，我需要這個生日，以便最老的一個獲得參觀。

Table 1: Data of the employees SEN = Date of Entry, BIR = Birthday

Table 2: Tours

Table 3: Requests with the tourid as well as the employeeid

這裏是我的查詢：

SELECT T.*, ER.SEN, requestcnt FROM tours T LEFT OUTER JOIN ( SELECT 
R.tourid, count(R.requestid) AS requestcnt, MIN(E.SEN) AS SEN 
     FROM requests R 
     INNER JOIN employees E ON E.employeeid = R.employeeid WHERE Funktion = 'XY' 
     GROUP by R.tourid 
     ) SR ON ER.tourid = U.tourid 

如何獲取到該遊最低資歷相匹配的員工的生日。如果我只是將生日字段添加到秒選語句中，即使他/她沒有請求旅程，我也會獲得總體最低年齡的員工的生日。

我們的目標是將所有內容都放在一個查詢中，因爲導遊的數量很大。此查詢適用於概覽頁面，因此我可以根據行程的可用性顯示錶情符號。

感謝您的任何建議！

Answer 1

可能對請求和員工進行另一次連接。

事情是這樣的： -

SELECT T.*, ER2.SEN, ER2.BIR, requestcnt 
FROM tours T 
LEFT OUTER JOIN 
(
    SELECT R.tourid, count(R.requestid) AS requestcnt, MIN(E.SEN) AS SEN 
    FROM requests R 
    INNER JOIN employees E ON E.employeeid = R.employeeid 
    WHERE Funktion = 'XY' 
    GROUP by R.tourid 
) ER 
ON ER.tourid = U.tourid 
LEFT OUTER JOIN 
(
    SELECT R.tourid, E.SEN, MIN(E.BIR) AS BIR 
    FROM requests R 
    INNER JOIN employees E ON E.employeeid = R.employeeid 
    WHERE Funktion = 'XY' 
    GROUP by R.tourid, E.SEN 
) ER2 
ON ER2.tourid = U.tourid 
AND ER.SEN = ER2.SEN 

編輯 - 對於修訂後的要求

獲取任何東西的3是尷尬的，因爲你不能真正使用MIN/MAX功能得到它（用很多努力，你可以通過使用它們來找到並排除min，然後再用這個結果等等）。對於連接的子查詢，您無法輕鬆使用限制，因爲它可以處理查詢的整體結果，而不是每個組。

2種方式我可以想到我的頭頂。首先將一個序列號添加到子查詢中，該子查詢將按照資歷順序得到結果，這將重新設置三元組更改。

像這樣（未測試）： -

SELECT T.*, ER2.SEN, ER2.BIR, requestcnt 
FROM tours T 
LEFT OUTER JOIN 
(
    SELECT R.tourid, count(R.requestid) AS requestcnt 
    FROM requests R 
    INNER JOIN employees E ON E.employeeid = R.employeeid 
    WHERE Funktion = 'XY' 
    GROUP by R.tourid 
) ER 
ON ER.tourid = T.tourid 
LEFT OUTER JOIN 
(
    SELECT R.tourid, E.employeeid, E.SEN, E.BIR, @seq:=IF(R.tourid=@tourid, @seq+1, 1) AS seq 
    FROM requests R 
    INNER JOIN employees E ON E.employeeid = R.employeeid 
    CROSS JOIN (SELECT @seq:=0, @tourid:=0) 
    WHERE Funktion = 'XY' 
    ORDER BY R.tourid, E.SEN, E.BIR, E.employeeid 
) ER2 
ON ER2.tourid = T.tourid 
AND ER2.seq = 3 

另一種方法是使用相關子查詢的SELECT。這可能會顯着放慢的行數返回的增加： -

SELECT T.*, 
     (
      SELECT E.SEN 
      FROM requests R 
      INNER JOIN employees E ON E.employeeid = R.employeeid 
      WHERE Funktion = 'XY' 
      AND R.tourid = T.tourid 
      ORDER BY E.SEN, E.BIR 
      LIMIT 3,1 
     ), 
     (
      SELECT E.BIR 
      FROM requests R 
      INNER JOIN employees E ON E.employeeid = R.employeeid 
      WHERE Funktion = 'XY' 
      AND R.tourid = T.tourid 
      ORDER BY E.SEN, E.BIR 
      LIMIT 3,1 
     ), 
     requestcnt 
FROM tours T 
LEFT OUTER JOIN 
(
    SELECT R.tourid, count(R.requestid) AS requestcnt 
    FROM requests R 
    INNER JOIN employees E ON E.employeeid = R.employeeid 
    WHERE Funktion = 'XY' 
    GROUP by R.tourid 
) ER 
ON ER.tourid = T.tourid 

或可能在WHERE子句中（同樣未測試）

SELECT T.*, EM.SEN, EM.BIR, EM.employeeid, 
     requestcnt 
FROM tours T 
CROSS JOIN employees EM 
LEFT OUTER JOIN 
(
    SELECT R.tourid, count(R.requestid) AS requestcnt 
    FROM requests R 
    INNER JOIN employees E ON E.employeeid = R.employeeid 
    WHERE Funktion = 'XY' 
    GROUP by R.tourid 
) ER 
ON ER.tourid = T.tourid 
WHERE EM.employeeid = 
(
    SELECT E.employeeid 
    FROM requests R 
    INNER JOIN employees E ON E.employeeid = R.employeeid 
    WHERE Funktion = 'XY' 
    AND R.tourid = T.tourid 
    ORDER BY E.SEN, E.BIR 
    LIMIT 3,1 
)