如果在Microsoft tSQL中找不到行，则返回一个值

使用Microsoft版本的SQL，这是我的简单查询。如果查询一条不存在的记录，则不会返回任何内容。我希望在这种情况下返回false（0）。寻找最简单的方法来说明没有记录。

SELECT  CASE
            WHEN S.Id IS NOT NULL AND S.Status = 1 AND (S.WebUserId = @WebUserId OR S.AllowUploads = 1) THEN 1
            ELSE 0
        END AS [Value]

        FROM Sites S

        WHERE S.Id = @SiteId

SELECT CASE WHEN COUNT(1) > 0 THEN 1 ELSE 0 END AS [Value]

FROM Sites S

WHERE S.Id = @SiteId and S.Status = 1 AND 
      (S.WebUserId = @WebUserId OR S.AllowUploads = 1)

这类似于Adam Robinson的，但是使用ISNULL而不是COUNT。

SELECT ISNULL(
(SELECT 1 FROM Sites S
WHERE S.Id = @SiteId and S.Status = 1 AND 
      (S.WebUserId = @WebUserId OR S.AllowUploads = 1)), 0)

如果内部查询具有匹配的行，则返回1。然后，外部查询（带有ISNULL）返回此值1。如果内部查询没有匹配的行，则不返回任何内容。外部查询将其视为NULL，因此ISNULL最终返回0。

这可能是一匹老马，当不存在任何行时返回1行的另一种方式是对UNION进行另一个查询并在表中不存在时显示结果。

SELECT S.Status, COUNT(s.id) AS StatusCount
FROM Sites S
WHERE S.Id = @SiteId
GROUP BY s.Status
UNION ALL --UNION BACK ON TABLE WITH NOT EXISTS
SELECT 'N/A' AS Status, 0 AS StatusCount
WHERE NOT EXISTS (SELECT 1
   FROM Sites S
   WHERE S.Id = @SiteId
) 

就像是：

if exists (select top 1 * from Sites S where S.Id IS NOT NULL AND S.Status = 1 AND (S.WebUserId = @WebUserId OR S.AllowUploads = 1))
    select 1
else
    select 0

我在这里阅读了所有答案，花了一段时间才弄清楚发生了什么。以下是根据Moe Sisko的回答和一些相关研究得出的

如果您的SQL查询不返回任何数据，则不存在具有null值的字段，因此ISNULL和COALESCE均无法按您希望的那样工作。通过使用子查询，顶级查询将获得一个具有空值的字段，并且ISNULL和COALESCE均可按您希望/期望的方式工作。

我的查询

select isnull(
 (select ASSIGNMENTM1.NAME
 from dbo.ASSIGNMENTM1
 where ASSIGNMENTM1.NAME = ?)
, 'Nothing Found') as 'ASSIGNMENTM1.NAME'

我的查询与评论

select isnull(
--sub query either returns a value or returns nothing (no value)
 (select ASSIGNMENTM1.NAME
 from dbo.ASSIGNMENTM1
 where ASSIGNMENTM1.NAME = ?)
 --If there is a value it is displayed 
 --If no value, it is perceived as a field with a null value, 
 --so the isnull function can give the desired results
, 'Nothing Found') as 'ASSIGNMENTM1.NAME'

您只需要用LEFT JOIN替换WHERE：

SELECT  CASE
        WHEN S.Id IS NOT NULL AND S.Status = 1 AND ...) THEN 1
        ELSE 0
    END AS [Value]

    FROM (SELECT @SiteId AS Id) R
    LEFT JOIN Sites S ON S.Id = R.Id

此解决方案使您还可以返回每列的默认值，例如：

SELECT
    CASE WHEN S.Id IS NULL THEN 0 ELSE S.Col1 END AS Col1,
    S.Col2,
    ISNULL(S.Col3, 0) AS Col3
FROM
    (SELECT @Id AS Id) R
    LEFT JOIN Sites S ON S.Id = R.Id AND S.Status = 1 AND ...

没有匹配的记录意味着没有记录返回。如果未找到任何记录，则“ 0”的值无处可走。您可以创建一个疯狂的UNION查询来执行所需的操作，但是只需检查结果集中的记录数，就可以做得更好。

这可能是一种方式。

SELECT TOP 1 [Column Name] FROM (SELECT [Column Name] FROM [table]
    WHERE [conditions]
    UNION ALL
    SELECT 0 )A ORDER BY [Column Name] DESC

带领带呢？

SELECT TOP 1 WITH TIES tbl1.* FROM 
        (SELECT CASE WHEN S.Id IS NOT NULL AND S.Status = 1 
                      AND (S.WebUserId = @WebUserId OR 
                           S.AllowUploads = 1)
                     THEN 1 
                     ELSE 0 AS [Value]
         FROM Sites S
         WHERE S.Id = @SiteId) as tbl1
ORDER BY tbl1.[Value]

DECLARE @col int; 
select @col = id  FROM site WHERE status = 1; 
select coalesce(@col,0);

@ hai-phan的答案使用LEFT JOIN是关键，但是可能很难遵循。我有一个复杂的查询，也可能不返回任何内容。我只是简化了他对我的需求的回答。应用于具有许多列的查询很容易。

;WITH CTE AS (
  -- SELECT S.Id, ...
  -- FROM Sites S WHERE Id = @SiteId
  -- EXCEPT SOME CONDITION.
  -- Whatever your query is
)
SELECT CTE.* -- If you want something else instead of NULL, use COALESCE.
FROM (SELECT @SiteId AS ID) R
LEFT JOIN CTE ON CTE.Id = R.ID

更新：SqlServerCentral上的此答案是最好的。它利用了MAX的这一特性- “当没有可供选择的行时，MAX返回NULL”。

SELECT ISNULL(MAX(value), 0) FROM table WHERE Id = @SiteId

You should avoid using expensive methods. You dont need any column for TBL2. 

SELECT COUNT(*) FROM(
         SELECT TOP 1     1 AS CNT  FROM  TBL1 
         WHERE ColumnValue ='FooDoo'  ) AS TBL2

or

IF EXISTS (SELECT TOP 1 1 FROM TABLE1 AS T1 
                          WHERE T1.ColumnValue='VooDoo') 
   SELECT 1 
ELSE 
   SELECT 0