如果没有找到记录，SQL select 返回 0，否则返回值

Question

我现在使用 Microsoft SQL，我的代码是：

   SELECT TOP 1 
   [avail]
   FROM [table1]
   where [name] = 'abc'
   order by [datetime] desc

我希望当 [avail] 存在时，返回 [avail] 的值，如果不存在，则返回 0 或“Not Found”

谢谢！

Answer 1

你可以用这个

SELECT ISNULL(( SELECT TOP 1 
   [avail]
   FROM [table1]
   where [name] = 'abc'
   order by [datetime] desc), 0) AS [avail]

Answer 2

您可以通过两种方式进行：

使用IF EXISTS方式：

IF EXISTS (SELECT TOP 1 [avail] FROM [table1] WHERE [name] = 'abc' ORDER BY [datetime] DESC)
    SELECT TOP 1 [avail] FROM [table1] WHERE [name] = 'abc' ORDER BY [datetime] DESC
ELSE 
    SELECT 0 AS [avail]

或者将avail值存储到一个变量中并进行处理，这种方式的唯一风险是如果top 1 avail返回空值，那么您将得到的结果为零。

DECLARE @Avail AS VARCHAR(100) = ''; -- varchar of the `avail` data size

SELECT TOP 1 @Avail = [avail] FROM [table1] WHERE [name] = 'abc' ORDER BY [datetime] DESC;

IF @Avail = ''
    SELECT 0 AS [avail]
ELSE 
    SELECT @Avail AS [avail]

Answer 3

尝试使用 COALESCE。 它从具有非空值的第一个参数中选择数据。 如果avail 不为空，返回[avale]，否则返回“Not Found”

SELECT COALESCE(avail, 'Not Found')
FROM table1
WHERE name = 'abc'
ORDER BY datetime desc

您可以在https://docs.microsoft.com/en-us/sql/t-sql/language-elements/coalesce-transact-sql 中阅读有关 COALESCE 的信息

Answer 4

如果它是您想要的前 1 名或删除 top 命令

 SELECT TOP 1 
   isnull([avail],'0')
   FROM [table1]
   where [name] = 'abc'
   order by [datetime] desc