Question

我想在sql查询中插入类似的内容：

Select * from Users where id=[if @userId>3 then @userId else "donnt use this condition"] and Name=[switch @userId  
case 1:"Alex"
case 2:"John"
default:"donnt use this condition"];

我该怎么做？

又一个类似的问题

当showAll为false时，它可以正常运行但是当showAll为true时它什么都不返回。为什么以及如何使其正常工作？ IsClosed列有一个位类型。

Select * from orders where IsClosed=CASE WHEN @showAll='false' THEN 'false' ELSE NULL END;

Answer 1

这将表现得非常糟糕：

Select * 
  from Users 
 where (@userid > 3 AND id = @userId)
    OR (@userId BETWEEN 1 AND 2 AND name = CASE 
                                             WHEN @userId = 1 THEN 'Alex' 
                                             ELSE 'John' 
                                           END)

表现最佳的选项是动态SQL：

SQL Server 2005 +

DECLARE @SQL NVARCHAR(4000)
    SET @SQL = 'SELECT u.*
                  FROM USERS u
                 WHERE 1 = 1 '

    SET@SQL = @SQL + CASE 
                       WHEN @userId > 3 THEN ' AND u.id = @userId '
                       ELSE ''
                     END

    SET@SQL = @SQL + CASE @userId
                       WHEN 1 THEN ' AND u.name = ''Alex'' '
                       WHEN 2 THEN ' AND u.name = ''John'' '
                       ELSE ''
                     END

BEGIN

 EXEC sp_executesql @SQL, N'@userId INT', @userId

END

有关SQL Server动态SQL支持的更多信息，请阅读“The Curse and Blessings of Dynamic SQL”

Answer 2

Select *
from Users
where id = CASE WHEN @userId>3 THEN @userId ELSE NULL END
OR name = CASE WHEN @userId = 1 THEN 'Alex' WHEN @UserId = 2 THEN 'John' ELSE NULL END

Answer 3

请试试这个：

select * 
from Users 
where id = (case when @userId > 3 then @userId 
                else id end)
and Name = (case cast(@userId as varchar)
                when '1' then 'Alex'
                when '2' then 'John'
                else Name end)

或者我认为这会更好：

select aa.*
from (select *
            , case when @userId > 3 then @userId 
                    else id end as UserID
            , case cast(@userId as varchar)
                    when '1' then 'Alex'
                    when '2' then 'John'
                    else Name end as UserName
        from Users) aa
where aa.id = aa.UserID
    and aa.Name = aa.UserName

您可能希望定义select上仅需要的每个字段，而不是使用星号（*）。

sql查询中的条件

3 个答案:

SQL Server 2005 +