ISNULL，IIF，CASE語句中的子查詢

Question

無論條件如何，在ISNULL或IIF或CASE中作為參數給出的子查詢都會被執行

為了解釋我的意思，請考慮以下示例。 當我運行以下查詢並查看執行計划時，我發現即使變量@Id不為NULL，第二個條件也會始終執行。

任何人都可以解釋為什么子查詢在查詢1,2,3中執行？

--Create a Temp table
IF OBJECT_ID('tempdb..#TempTable') IS NOT NULL DROP TABLE #TempTable
CREATE TABLE #TempTable
(
ID INT
)

--Insert some data
INSERT INTO #TempTable VALUES(1),(2),(3),(4),(5),(6),(7),(8),(9),(10)

DECLARE @Id INT = 1

--Query 1: ISNULL
SET @Id= ISNULL(@Id, (SELECT TOP 1 ID FROM #TempTable TT WHERE TT.ID = 1))
SELECT @Id AS ID

--Query 2: IIF
SET @Id=  IIF(@Id IS NULL,(SELECT TOP 1 ID FROM #TempTable TT WHERE TT.ID = 1),@Id)
SET @Id=  IIF(@Id IS NOT NULL,@Id,(SELECT TOP 1 ID FROM #TempTable TT WHERE TT.ID = 1))

SELECT @Id AS ID

--Query 3: CASE
SET @Id=  CASE WHEN @Id IS NULL THEN (SELECT TOP 1 ID FROM #TempTable TT WHERE TT.ID = 1) ELSE @Id END
SET @Id=  CASE WHEN @Id IS NOT NULL THEN @Id ELSE (SELECT TOP 1 ID FROM #TempTable TT WHERE TT.ID = 1)  END
SELECT @Id AS ID

---Query 4: IF
IF @Id IS NULL
BEGIN
    SET @Id = (SELECT TOP 1 ID FROM #TempTable TT WHERE TT.ID = 1) 
    SELECT @Id AS ID
END
ELSE 
BEGIN
    SELECT @Id AS ID
END

Answer 1

如果希望子查詢僅在滿足空條件時執行，請使用IF而不是ISNULL

if @Id is null
   set @id = (SELECT TOP 1 ID FROM #TempTable TT WHERE TT.ID = 1)

代替

SET @Id= ISNULL(@Id, (SELECT TOP 1 ID FROM #TempTable TT WHERE TT.ID = 1))