SQL Server：Subselect IS NULL NULL'VALUE'

Question

我试图在SQL Server中返回一个值为IS NULL的字符串结果。

我正在做的一个例子如下。这是一个子选择查询...

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 

不管我做什么，我不能让子选择的输出结果改变，如果结果是NULL。并非所有SELECT语句返回的结果都会返回结果，这是我需要将输出设置为“NO LEVEL”的地方。子选择将永远不会返回多个结果。

希望这是有道理的。

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; 

我怀疑，虽然，你的问题不在于remaining是NULL，但没有匹配。您可以移动COALESCE()子查询外，但我会提出一个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;