关键字'IF'附近的语法不正确

Question

我试图将IF .. ELSE放在WHERE子句中，如下所示，但收到错误消息

“关键字'IF'附近的语法不正确。'''附近的语法不正确。”

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

您不能以这种方式在子选择中使用IF 。 而是使用AND来测试WHERE子句中变量@categoryID的值。 这将需要两个()组，它们之间具有逻辑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放在像这样的SQL语句的中间。 有几种方法可以实现这一目标：

--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))