sql查询中的条件

我想在sql查询中插入如下内容：

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"];

我怎么做

还有一个类似的问题

当showAll为false时，它工作正常，但当showAll为true时，它不返回任何内容。为什么以及如何使其正常工作？IsClosed列具有位类型

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

---

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)

性能最好的选项是动态SQL：

SQL Server 2005+ 有关SQL Server动态SQL支持的详细信息，请阅读“”

请尝试以下操作：

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)

或者我认为这会表现得更好：

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

您可能希望定义仅在

选择中需要的每个字段，而不是使用星号（
*
）

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)

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