根据列条件连接两个表

Question

在 MS SQL Server 中，有两个表，“T1”和“T2”，有一个公共字段“ID (PK)”，其中 ID 是唯一的，但对于相同的 ID 记录，T1 和 T2 中的日期可能不同，ABC 或 DEF 的内容也可能不同 E.g:

----------
T1:
----------
ID   Date     A     B      C
----------
55   12/1/17  Jim  Smith  Male
77   12/2/17  Jim  Green  Male
99   10/3/17  Kate Lee    Male

T2:
----------
ID   Date       D    E     F
----------
55   12/1/17  Jim  Smith  Male
77   10/2/16  James  Jr   Male
99   12/25/17 Kathy  Lee  Male

我想要一个基于 ID 的某种形式的连接表，并根据最新日期选择 ABC 或 DEF，在这种情况下，我想要

----------
ID   Date     join1| join2 | join3
----------
55   12/1/17  Jim  Smith  Male
77   12/2/17  Jim  Green  Male
99   12/25/17 Kathy Lee   Male

这可能吗？

到目前为止，我想出的东西有点乏味：我首先做了一个完整的外部连接：

select * from T1 full outer join  T2 on T1.ID = T2.ID

具备以下条件：

---

ID   Date     A     B      C     Date2     D       E       F
----------
55   12/1/17  Jim  Smith  Male   12/1/17  Jim    Smith   Male
77   12/2/17  Jim  Green  Male   10/2/16  James   Jr     Male
99   10/3/17  Kate Lee    Male   12/25/17 Kathy   Lee    Male

然后我尝试使用case when then else选择ABC或DEF字段，看起来很丑，我担心性能。

select T1.ID, 
        case when T1.Date > T2.Date then T1.Date else T2.Date END as Date 
        case when T1.Date > T2.Date then T1.A else T2.D END as Join1
        case when T1.Date > T2.Date then T1.B else T2.E END as Joni2
        case when T1.Date > T2.Date then T1.C else T2.F END as Join3
from RESULT_TABLE

是否可以执行以下操作：

    select T1.ID,
           case when T1.Date > T2.Date then 
                T1.Date, T1.A as Join1, T1.B as Join2, T1.C as Join3
           else
                T2.Date, T2.D as Join1 , T2.E as Join2, T2.F as Join3
           END 
from RESULT_TABLE

Answer 1

一种方法使用case：

select t1.id,
       (case when t1.date >= t2.date then t1.date else t2.date end) as date,
       (case when t1.date >= t2.date then t1.a else t2.a end) as a,
       (case when t1.date >= t2.date then t1.b else t2.a end) as b,
       (case when t1.date >= t2.date then t1.c else t2.a end) as c
from t1 join
     t2
     on t1.id = t2.id;

如果你不喜欢所有的case 表达式，你可以这样做：

select t1.*
from t1 join
     t2
     on t1.id = t2.id and t1.date >= t2.date
union all
select t2.*
from t1 join
     t2
     on t1.id = t2.id and t1.date < t2.date;

注意：您可能希望列出列而不是使用select *。此版本假定两个表具有完全相同的列、以相同的顺序、具有兼容的类型。

Answer 2

当然，

Select t1.Id, 
  case when t1.Date > t2.Date then t1.Date else t2.Date end date,
  case when t1.Date > t2.Date then t1.A else t2.D end AD,
  case when t1.Date > t2.Date then t1.B else t2.E end BE,
  case when t1.Date > t2.Date then t1.C else t2.F end CF

From T1 join T2 on t2.Id = t1.Id