從Select語句中刪除子查詢？

Question

我有這個查詢，我需要從3個表中拉回數據，但有一些需要發生的分組。我知道子查詢是可怕的，但我似乎無法以其他方式完成此任務。有幾萬行時，這將會崩潰。也許我過於複雜了？我嘗試了「分區依據」，但是這將會撤回ERT1中的所有行，這會弄亂計數。我不能使用一個組，因爲如果我在ERT1上進行分組，它會搞砸ER1。

我在連接字段上使用FK和索引。

任何人有任何想法？

這將是非常感謝！這是非常酷 -

SELECT ER.EventRegistrationID, DC.DiscountCodeID, DC.DiscountAmount, DC.DiscountCode, DC.ActiveFlag, 

    ISNULL((SELECT COUNT(ER1.EventRegistrationID) 
     FROM T_EventRegistration ER1 
     WHERE ER1.DiscountCode = DC.DiscountCode 
     AND ER1.EventID = DC.EventID 
     AND ER1.OrderCompleteFlag = 1),0) AS NumTimesUsed, --includes refunded orders 

    ISNULL((SELECT COUNT(ERT1.TicketNumber) 
     FROM T_EventRegistrationTickets ERT1, T_EventRegistration ER1 
     WHERE ER1.EventRegistrationID = ERT1.EventRegistrationID 
     AND ER1.DiscountCode = DC.DiscountCode 
     AND ER1.EventID = DC.EventID 
     AND ER1.OrderCompleteFlag = 1 
     ),0) AS NumTicketsSold --excludes refunded tickets 

FROM T_DiscountCodes DC, T_EventRegistration ER 
WHERE DC.DiscountCode *= ER.DiscountCode 
AND DC.EventID = 20188 
AND DC.DeleteFlag = 0 
AND DC.HiddenFlag = 0 
AND ER.OrderCompleteFlag = 1 
ORDER BY DC.DiscountCode, NumTimesUsed 

Answer 1

您可以通過使用APPLY或通過創建派生表

SELECT ER.EventRegistrationID 
    , DC.DiscountCodeID 
    , DC.DiscountAmount 
    , DC.DiscountCode 
    , DC.ActiveFlag 
    , ISNULL(a,NumTimesUsed,0) 
    , ISNULL(a2.NumTicketsSold,0) 
FROM T_DiscountCodes DC, T_EventRegistration ER 
OUTER APPLY (SELECT COUNT(ERT1.TicketNumber) NumTimesUsed 
     FROM T_EventRegistrationTickets ERT1, T_EventRegistration ER1 
     WHERE ER1.EventRegistrationID = ERT1.EventRegistrationID 
     AND ER1.DiscountCode = DC.DiscountCode 
     AND ER1.EventID = DC.EventID 
     AND ER1.OrderCompleteFlag = 1) a 
OUTER APPLY (SELECT COUNT(ER1.EventRegistrationID) NumTicketsSold 
     FROM T_EventRegistration ER1 
     WHERE ER1.DiscountCode = DC.DiscountCode 
     AND ER1.EventID = DC.EventID 
     AND ER1.OrderCompleteFlag = 1) a2 
WHERE DC.DiscountCode *= ER.DiscountCode 
AND DC.EventID = 20188 
AND DC.DeleteFlag = 0 
AND DC.HiddenFlag = 0 
AND ER.OrderCompleteFlag = 1 
ORDER BY DC.DiscountCode, NumTimesUsed