爲什麼我不能在select語句中使用if語句？有誰知道另一種方法？

Select b.ItemKey, a.CatKey, a.ParentKey, a.CatName, g.ItemID, c.UserFld1, c.UserFld3, c.UserFld5, f.UOMID, e.SizeValue 
From dbo.timWebCategory a Inner JOIN 
    dbo.timWebCategoryItem b ON a.CatKey = b.CatKey Inner JOIN 
    mas500_app.dbo.timItem c ON b.ItemKey = c.ItemKey Inner JOIN 
    dbo.cpsIMItemDetails d ON c.ItemKey = d.ItemKey Inner JOIN 
    dbo.timItemSize e ON c.ItemKey = e.ItemKey Left JOIN 
    dbo.timVolumeUOM f ON e.VolUOMKey = f.VolUOMKey Left JOIN 
    mas500_app.dbo.vWebItem g ON c.ItemKey = g.ItemKey 
Where a.Catkey = @Key 
AND d.IntFlg = 1 
If @color is not Null 
Begin 
    AND c.UserFld1 = @color 
End 
Order By f.UOMID, e.SizeValue**

返回：關鍵字'AND'附近的語法不正確。爲什麼我不能在select語句中使用if語句？有誰知道另一種方法？

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2011-08-11 sidriz

（謂詞演算）的一般規則是：如果x然後y'在邏輯上等於'（NOT x）或y'。或者更正式地說，'p - > q =〜p v q'。 – Josh

試一下......

Select b.ItemKey, a.CatKey, a.ParentKey, a.CatName, g.ItemID, c.UserFld1, c.UserFld3, c.UserFld5, f.UOMID, e.SizeValue 
From dbo.timWebCategory a Inner JOIN 
    dbo.timWebCategoryItem b ON a.CatKey = b.CatKey Inner JOIN 
    mas500_app.dbo.timItem c ON b.ItemKey = c.ItemKey Inner JOIN 
    dbo.cpsIMItemDetails d ON c.ItemKey = d.ItemKey Inner JOIN 
    dbo.timItemSize e ON c.ItemKey = e.ItemKey Left JOIN 
    dbo.timVolumeUOM f ON e.VolUOMKey = f.VolUOMKey Left JOIN 
    mas500_app.dbo.vWebItem g ON c.ItemKey = g.ItemKey 
Where a.Catkey = @Key 
AND d.IntFlg = 1 
AND (@color = c.UserFld1 OR @color is null) 
Order By f.UOMID, e.SizeValue**

來源

2011-08-11 22:53:43 Chains

把這一「內而外」。你不能有條件地插入AND，你必須總是說AND，然後你可以使用if-else（或者只是一個OR，就像其他的答案）一樣有條件地提供你想要的條件或只是真/假。

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2011-08-11 22:54:12 Jesper

您可以使用if語句來控制執行哪些語句，但不能用它來刪除部分查詢。

不以這種方式構建的一個原因是，查詢的執行計劃會因輸入而有所不同，因此每次都必須創建新的執行計劃並且無法重新使用。

有不同的方式來處理這個問題，喜歡用isnull函數的值進行比較，以本身如果varaible爲null：

AND c.UserFld1 = isnull(@color, c.UserFld1)

或檢查空比較之前：

AND (@coor is null or c.UserFld1 = @color)

您始終可以在查詢周圍放置if語句，這會爲您提供查詢的兩個版本，每個版本的計劃比運行查詢時檢查該值的計劃略好。有時值得重複的代碼：

If @color is not Null Begin 

    Select b.ItemKey, a.CatKey, a.ParentKey, a.CatName, g.ItemID, c.UserFld1, c.UserFld3, c.UserFld5, f.UOMID, e.SizeValue 
    From dbo.timWebCategory a Inner JOIN 
    dbo.timWebCategoryItem b ON a.CatKey = b.CatKey Inner JOIN 
    mas500_app.dbo.timItem c ON b.ItemKey = c.ItemKey Inner JOIN 
    dbo.cpsIMItemDetails d ON c.ItemKey = d.ItemKey Inner JOIN 
    dbo.timItemSize e ON c.ItemKey = e.ItemKey Left JOIN 
    dbo.timVolumeUOM f ON e.VolUOMKey = f.VolUOMKey Left JOIN 
    mas500_app.dbo.vWebItem g ON c.ItemKey = g.ItemKey 
    Where a.Catkey = @Key 
    AND d.IntFlg = 1 
    AND c.UserFld1 = @color 
    Order By f.UOMID, e.SizeValue 

End Else Begin 

    Select b.ItemKey, a.CatKey, a.ParentKey, a.CatName, g.ItemID, c.UserFld1, c.UserFld3, c.UserFld5, f.UOMID, e.SizeValue 
    From dbo.timWebCategory a Inner JOIN 
    dbo.timWebCategoryItem b ON a.CatKey = b.CatKey Inner JOIN 
    mas500_app.dbo.timItem c ON b.ItemKey = c.ItemKey Inner JOIN 
    dbo.cpsIMItemDetails d ON c.ItemKey = d.ItemKey Inner JOIN 
    dbo.timItemSize e ON c.ItemKey = e.ItemKey Left JOIN 
    dbo.timVolumeUOM f ON e.VolUOMKey = f.VolUOMKey Left JOIN 
    mas500_app.dbo.vWebItem g ON c.ItemKey = g.ItemKey 
    Where a.Catkey = @Key 
    AND d.IntFlg = 1 
    Order By f.UOMID, e.SizeValue 

End

來源

2011-08-11 23:01:44 Guffa

爲什麼我不能在select語句中使用if語句？有誰知道另一種方法？

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