从联合中选择案例答案

【问题标题】：Select Case FROM Union从联合中选择案例
【发布时间】：2017-10-13 10:47:40
【问题描述】：

为了报告和创建报告（MS SQL SERVER 2014），它将比较最后一天、上周同一天和上个月同一天的交易数量和金额，我需要一些像这样的查询，但它不起作用：

Select 
   Acquirer = case when Date = @Day then a.Acquirer
                   when Date = @LastWeekDay then b.Acquirer
                   when Date = @LastMonthDay then c.Acquirer
              end
,  RC =       case when Date = @Day then a.RC
                   when Date = @LastWeekDay then b.RC
                   when Date = @LastMonthDay then c.RC
              end
, Desc =      case when Date = @Day then a.Desc
                   when Date = @LastWeekDay then b.Desc
                   when Date = @LastMonthDay then c.Desc
              end
, Day_Tr_Count =  case when Date = @Day then a.Count end
, LWD_Tr_Count =  case when Date = @LastWeekDay then b.Count end
, LMD_Tr_Count =  case when Date = @LastMonthDay then c.Count end
, Day_Tr_Amount =  case when Date = @Day then a.Amount end
, LWD_Tr_Amount =  case when Date = @LastWeekDay then b.Amount end
, LMD_Tr_Amount =  case when Date = @LastMonthDay then c.Amount end

from
  ( select * from T1
     union
    select * from T2
     union
    select * from T3) x
group by Acquirer,RC,Desc

所有 3 个表的结构都非常相似：

T1,T2,T3(acquirer,RC,Desc,Count,Amount,Date).

报告应该是：

result_table(Acquirer,RC,Desc,Daily_Count,Last_Week_Day_Count,Last_Month_Count,Daily_Amount,Last_Week_Day_Amount,Last_Month_Day_Amount)

【问题讨论】：

第 1 步：您可能想要UNION ALL。
如何不工作？
一般 GROUP BY 规则说：“如果指定了 GROUP BY 子句，则 SELECT 列表中的每个列引用必须标识一个分组列或作为集合函数的参数。”
因为他在查询别名中没有a、b、c表
我不明白为什么人们会写出带有这么多基本错误的庞大查询...他们应该一步一步地这样做，以防他们不知道自己在做什么。

标签： sql sql-server select case union

【解决方案1】：

嗯。您有一个没有group by 的聚合查询。也许：

Select v.Acquirer, v.RC, v.Desc,
       Day_Tr_Count =  sum(case when Date = @Day then a.Count end),
       LWD_Tr_Count =  sum(case when Date = @LastWeekDay then b.Count end),
       LMD_Tr_Count =  sum(case when Date = @LastMonthDay then c.Count end),
       Day_Tr_Amount =  sum(case when Date = @Day then a.Amount end
       LWD_Tr_Amount =  sum(case when Date = @LastWeekDay then b.Amount end),
       LMD_Tr_Amount =  sum(case when Date = @LastMonthDay then c.Amount end)

from (select * from T1
      union all
      select * from T2
      union all
      select * from T3
     ) x cross apply
     (values (case when Date = @Day then a.Acquirer
                   when Date = @LastWeekDay then b.Acquirer
                   when Date = @LastMonthDay then c.Acquirer
              end,
              case when Date = @Day then a.RC
                   when Date = @LastWeekDay then b.RC
                   when Date = @LastMonthDay then c.RC
              end,
              case when Date = @Day then a.Desc
                   when Date = @LastWeekDay then b.Desc
                   when Date = @LastMonthDay then c.Desc
              end
             ) v (Acquirer, RC, Desc)
group by v.Acquirer, v.RC, v.Desc;

注意：desc 是列（或其他任何内容）的错误名称。它是一个 SQL 关键字（想想order by）。

【讨论】：