理解SQL查詢的指南

Question

在過去一周左右的時間里，我一直在嘗試使用以下查詢作為參考來創建查詢，並且需要一些幫助來分解它，因此我知道每個部分的工作。

Select Distinct
        tblCustomers.fldEmail
      , tblCustomers.fldCustomerID
      , tblCustomers.fldName
      , tblCustomers.fldContactName
From    tblBigClubs
Inner Join tblBigClubMatch
        On tblBigClubs.fldBigClubID = tblBigClubMatch.fldBigClubID
Inner Join tblCustomers
        On tblBigClubMatch.fldCustomerID = tblCustomers.fldCustomerID
Where   (tblBigClubs.fldBigClubID In (Select Distinct
                                                dbo.tblBigClubMatch.fldBigClubID
                                      From      dbo.tblOrders
                                      Inner Join dbo.tblOrderItems
                                                On dbo.tblOrders.fldOrderID = dbo.tblOrderItems.fldOrderID
                                      Inner Join dbo.tblBigClubMatch
                                                On dbo.tblOrders.fldCustomerID = dbo.tblBigClubMatch.fldCustomerID
                                      Where     (dbo.tblOrders.fldOrderDate > DateAdd(Year, -2, GetDate()))
                                                And (dbo.tblOrderItems.fldStockCode Like 'TO%')
                                      Group By  dbo.tblBigClubMatch.fldBigClubID
                                      Having    (Sum(dbo.tblOrderItems.fldQtyOrder) >= 30))
         And tblCustomers.fldEmail Is Not Null
         And dbo.tblCustomers.fldEmail <> ''
         And Len(dbo.tblCustomers.fldEmail) > 8
        );

請有人可以解釋每個部分在做什么嗎？

Answer 1

首先，簡化並格式化：

Select Distinct c.fldEmail, c.fldCustomerID, c.fldName, c.fldContactName
From tblBigClubs b
   Join tblBigClubMatch m On m.fldBigClubID = b.fldBigClubID 
   Join tblCustomers c On c.fldCustomerID = m.fldCustomerID
Where Len(c.fldEmail) > 8
   and b.fldBigClubID In 
      (Select Distinct im.fldBigClubID
       From dbo.tblOrders o
            Join dbo.tblOrderItems i On i.fldOrderID = o.fldOrderID
            Join dbo.tblBigClubMatch im On im.fldCustomerID = o.fldCustomerID
       Where o.fldOrderDate > DateAdd(Year, -2, GetDate())
           And i.fldStockCode Like 'TO%'
       Group By  im.fldBigClubID
       Having Sum(i.fldQtyOrder) >= 30);

以上在功能上是等效的。 （使用別名消除了很多混亂，並使SQL更易於閱讀和理解），然后檢查謂詞。 只有兩個：

Len(c.fldEmail) > 8如果滿足，則顯然fldEmail不能為null，並且

b.fldBigClubID In subquery子句中。 看起來好像正在查找fldBigClubID所有值，其中fldStockCode以TO開頭，而fldOrderDate在最近兩年中，在同一時期內有30個或更多訂單。

總而言之，對於過去兩年中具有30個或更多訂單的股票代碼為TO商品的客戶，似乎從客戶表中獲得了四個字段。

所以實際上，這也應該等同於：

Select fldEmail, fldCustomerID, 
   fldName, fldContactName
From tblCustomers 
Where fldCustomerID In
   (Select distinct o.fldCustomerID
      From dbo.tblOrders o
        Join dbo.tblOrderItems i On i.fldOrderID = o.fldOrderID
        Join dbo.tblBigClubMatch m On m.fldCustomerID = o.fldCustomerID
    Where o.fldOrderDate > DateAdd(Year, -2, GetDate())
       And i.fldStockCode Like 'TO%'
    Group By m.fldBigClubID
    Having Sum(i.fldQtyOrder) >= 30);

Answer 2

不可能遍歷整個查詢，但是如果您遇到麻煩的特定部分，我們可以為您提供幫助。 回應您的評論

DateAdd(Year, -2, GetDate())

返回的日期是2年減去今天的日期。 你可以跑

SELECT DateAdd(Year, -2, GetDate())

退貨

2014-12-07 10:56:07.290

因此，看起來他們正在吸引過去兩年中訂購30件或更多商品的客戶。