SQL Server:案例中的多个操作

Question

我将此案例作为更大的SQL语句的一部分:

.
.
.
CASE    
   WHEN [AEC].item = 'S' THEN [sOld].code
   WHEN [AEC].item= 'R' THEN [rOld].code
END AS [oldCode],

CASE    
   WHEN [AEC].item= 'S' THEN [sOld].system
   WHEN [AEC].item= 'R' THEN [rOld].system
END AS [oldStatusSystem],

CASE    
   WHEN [AEC].item= 'S' THEN [sNew].code
   WHEN [AEC].item= 'R' THEN [rNew].code
END AS [newCode],

CASE     
   WHEN [AEC].item= 'S' THEN [sNew].system
   WHEN [AEC].item= 'R' THEN [rNew].system
END AS [newStatusSystem],
.
.

LEFT JOIN 
    tblSystemStatuses AS [sOld] ON [AEC].oldValue = [sOld].status
LEFT JOIN
    tblSystemStatuses AS [sNew] ON [AEC].newValue = [sNew].status
LEFT JOIN
    tblSystemRoles AS [rOld] ON [AEC].oldValue = [rOld].role
LEFT JOIN
    tblSystemRoles AS [rNew] ON [AEC].newValue = [rNew].role

我想要的情况是这样的:

CASE    
   WHEN [AEC].item = 'S' 
      THEN 
           [sOld].code AS [oldCode],
           [sOld].system AS [oldStatusSystem],
           [sNew].code AS [newCode], 
           [sNew].system AS [newStatusSystem]
   ELSE WHEN [AEC].item = 'R' 
      THEN 
           [rOld].code AS [oldCode],
           [rOld].system AS [oldStatusSystem],
           [rNew].code AS [newCode], 
           [rNew].system AS [newStatusSystem]
  END

换句话说,我想作为then语句的一部分执行一些操作.我无法在任何地方看到一个例子.

我不能写if else因为我需要进行一些连接并依赖来自几个表的数据.有没有办法以我想要的方式写这个案子？

Answer 1

甲case表达式仅返回单个值.所以,你不能做你想做的事.

您可以简化逻辑,但方式略有不同.可能最好的方法是使用cross apply.请注意,这在from子句中,因为selectcan中的表达式只返回标量值.因此,您需要在选择中选择所需的列:

select v.oldcode, . . . 
from . . . cross apply
     (select v.*
      from (values ('S', [sOld].code, [sOld].system, [sNew].code, [sNew].system),
                   ('R', [rOld].code, [rOld].system, [rNew].code, [rNew].system)
           ) v(item, oldcode, oldsystem, newcode, newsystem)
      where v.item = [AEC].item
     ) v

这满足了与单个case表达式相同的需求.值得注意的是,匹配逻辑只出现一次.这个版本的优点是各个字段可以在多个表达式中使用select.