SQL Server：Subselect IS NULL NULL'VALUE'

Question

我試圖在SQL Server中返回一個值爲IS NULL的字符串結果。

我正在做的一個例子如下。這是一個子選擇查詢...

SELECT 
    stockitems.code AS StockCode, 
    stockitems.description AS 
ShortDescription, 
    (SELECT 
     CASE remaining 
      WHEN NULL 
       THEN 'NO LEVEL' 
      ELSE remaining 
     END 
    FROM StockItemAlertLevels 
    WHERE StockItems.ID = StockItemAlertLevels.StockItemID) AS RemainingLevel 
FROM 
    stockitems 
WHERE 
    StockItems.Attribute1 = '1' 
ORDER BY 
    StockItems.Code 

不管我做什麼，我不能讓子選擇的輸出結果改變，如果結果是NULL。並非所有SELECT語句返回的結果都會返回結果，這是我需要將輸出設置爲「NO LEVEL」的地方。子選擇將永遠不會返回多個結果。

希望這是有道理的。

Answer 1

我想你想要COALESCE()：

SELECT si.code As StockCode, si.description AS ShortDescription, 
     (SELECT COALESCE(remaining, 'NO LEVEL')  
     FROM StockItemAlertLevels sal 
     WHERE si.ID = sal.StockItemID 
     ) as RemainingLevel 
FROM stockitems si 
WHERE si.Attribute1 = '1' 
ORDER BY si.Code; 

我懷疑，雖然，你的問題不在於remaining是NULL，但沒有匹配。您可以移動COALESCE()子查詢外，但我會提出一個LEFT JOIN：

SELECT si.code As StockCode, si.description AS ShortDescription, 
     COALESCE(remaining, 'NO LEVEL') as RemainingLevel 
FROM stockitems si LEFT JOIN 
    StockItemAlertLevels sal 
    ON si.ID = sal.StockItemID 
WHERE si.Attribute1 = '1' 
ORDER BY si.Code;