關鍵字'IF'附近的語法不正確

Question

我試圖將IF .. ELSE放在WHERE子句中，如下所示，但收到錯誤消息

“關鍵字'IF'附近的語法不正確。'''附近的語法不正確。”

DECLARE @categoryID int

SET @categoryID = 0

SELECT * from SE_Auctions
  WHERE ItemCategoryID IN
  (
   IF @categoryID = 0
     SELECT CategoryID from SE_ItemCategory
   ELSE
     SELECT CategoryID from SE_ItemCategory 
    WHERE ParentID = @categoryID 
          OR CategoryID = @categoryID
  )

Answer 1

您不能以這種方式在子選擇中使用IF 。 而是使用AND來測試WHERE子句中變量@categoryID的值。 這將需要兩個()組，它們之間具有邏輯OR 。

WHERE 
-- First case, @categoryID = 0
(@categoryID = 0 AND ItemCategoryID IN (
  SELECT CategoryID from SE_ItemCategory
))
-- Other case, @categoryID <> 0
OR (@categoryID <> 0 AND ItemCategoryID IN (
  SELECT CategoryID from SE_ItemCategory 
    WHERE ParentID=@categoryID OR CategoryID=@categoryID
))

Answer 2

你不能把if放在像這樣的SQL語句的中間。 有幾種方法可以實現這一目標：

--Example 1
DECLARE @categoryID int = 0

SELECT * from SE_Auctions
WHERE 
ItemCategoryID IN
(SELECT CategoryID from SE_ItemCategory where @categoryID=0
union all
SELECT CategoryID from SE_ItemCategory 
    WHERE ParentID=@categoryID OR CategoryID=@categoryID) 

--Example 2
DECLARE @categoryID int = 0

SELECT * from SE_Auctions
WHERE 
(@categoryID=0
and ItemCategoryID IN
(SELECT CategoryID from SE_ItemCategory))
or (@categoryID <> 0
and ItemCategoryID IN
(SELECT CategoryID from SE_ItemCategory 
    WHERE ParentID=@categoryID OR CategoryID=@categoryID))

--Example 3
DECLARE @categoryID int = 0

SELECT * from SE_Auctions
WHERE 
ItemCategoryID IN
(SELECT CategoryID from SE_ItemCategory 
    WHERE ParentID=@categoryID OR CategoryID=@categoryID or @categoryID=0))