SQL在where子句中使用CASE

Question

这是一个表的例子：

     acIdent   |  acSubject          | acCode
     200.2.013 |                     | F-202
     200.2.013 | Minsk traktor works | F202.GRS

这是我试图做的：

     declare @subj varchar(30)

     set @subj = 'Minsk traktor works'

     select acIdent, acSubject, acCode
     from tHE_SetItemExtItemSubj where acIdent = '200.2.013' and
     acSubject = (
            case 
            when @subj = acSubject then @subj
            else '' end
         )

我的目标是只获得一条记录。我的例子返回两个记录。当@subj存在于表中然后返回acCode - F202.GRS，否则返回F-202。

请帮忙

Answer 1

如果我理解正确，你正在寻找这样的东西：

 declare @subj varchar(30)

 set @subj = 'Minsk traktor works'

 select top(1) acIdent, acSubject, acCode
 from tHE_SetItemExtItemSubj 
 where acIdent = '200.2.013' 
   and (acSubject = @subj or acSubject = '')
 order by len(acSubject) desc

Answer 2

我在上面编译了你的查询，如下所示：

declare @MyTable as table
    (   ID  int identity primary key
    ,   acIdent     varchar(100)
    ,   acSubject   varchar(200)
    ,   acCode      varchar(100)
    )

insert into @MyTable
    (   acIdent
    ,   acSubject
    ,   acCode  )
    values
    (   '200.2.013'
    ,   ''
    ,   'F-202' ),
    (   '200.2.013'
    ,   'Minsk traktor works'
    ,   'F-202' )

declare @subj varchar(30)

    set @subj = 'Minsk traktor works'

    select
        acIdent
    ,   acSubject
    ,   acCode
    ,   case acSubject when  @subj then @subj else '' end
    from @MyTable where acIdent = '200.2.013'
    and  acSubject = (case when acSubject = @subj then @subj else '' end)

    select
        acIdent
    ,   acSubject
    ,   acCode
    ,   case acSubject when  @subj then @subj else '' end
    from @MyTable where acIdent = '200.2.013'
    and  acSubject = @subj

从2个查询中，您会注意到第一个查询将始终为您提供2行，因为您的代码存在缺陷。

您将注意到的第二个查询只会给您一条记录。

结果将如下所示：

Results

Answer 3

这不会有用，因为你必须只使用一个条件获得一条记录。如果您必须仅针对单一条件进行检查，为什么要使用案例？

declare @subj varchar(30)
set @subj = 'Minsk traktor works'
select top(1) acIdent, acSubject, acCode
from tHE_SetItemExtItemSubj 
where acIdent = '200.2.013' 
and (acSubject = @subj)

Answer 4

通过案例功能可以做到这样的事情：

 -- assumes @subj is not '' nor null. 
 select y.acIdent, y.acSubject, y.acCode
 from tHE_SetItemExtItemSubj y
 where y.acIdent = '200.2.013' 
   and y.acSubject = 
   (case 
    when exists(select * from  tHE_SetItemExtItemSubj x where 
                x.acIdent = y.acIdent and x.acSubject = @subj) 
    then @subj else '' 
    end)

案例函数也可以嵌套。这是一个具有与上面相同功能的例子（在这种情况下它可能有点过分，但是当你做更复杂的事情时，这个技术会很有用）：

 -- assumes @subj is not '' nor null. 
 select y.acIdent, y.acSubject, y.acCode
 from tHE_SetItemExtItemSubj y
 where y.acIdent = '200.2.013' 
   and 1 = 
   (case 
    when exists(select * from  tHE_SetItemExtItemSubj x where 
                x.acIdent = y.acIdent and x.acSubject = @subj) 
    then 
        case when y.acSubject = @subj then 1 else 0 end
    else 
        case when y.acSubject = '' then 1 else 0 end
    end)