sql - t-sql 中的左外连接

Question

我有以下两个表。我使用的是 SQL Server 2008 R2

Create table #tmp1 (
a char(1)
)

Create table #tmp2 (
id int,
a char(1),
val int
)

insert #tmp1 values ('A')
insert #tmp1 values ('B')
insert #tmp1 values ('C')

insert #tmp2 values (1,  'A', 10)
insert #tmp2 values (1,  'B', 20)
insert #tmp2 values (2,  'A', 30)
insert #tmp2 values (2,  'C', 40)

select * from #tmp1 t1 left outer join #tmp2 t2 on t1.a = t2.a
order by t2.id

这将返回结果集

A   1   A   10
B   1   B   20
C   2   C   40
A   2   A   30

我想要以下结果集

 A     1    A       10
 B     1    B       20
 C     1    null    null
 A     2    A       30
 B     2    null    null
 C     2    C       40

现在，我正在通过创建一个具有这样的交叉联接的新表，然后执行外部联接来实现此目的

select * into #tmp3 from #tmp1 cross join (select distinct ID from #tmp2) t
select * from #tmp3 t1 left outer join #tmp2 t2 on t1.a = t2.a and t1.id = t2.id

有更好的方法吗？

谢谢

Answer 1

为了得到你想要的东西，你需要一张“驾驶” table 。也就是说，您需要所有组合的完整列表，然后连接到其他表以获取匹配项。这是一种方法:

select t1.a, t2.*
from (select t1.a as a, t2.id as id
      from (select distinct a from #tmp1 t1) t1
           cross join
           (select distinct id from #tmp2 t2) t2
     ) driving left outer join
     #tmp1 t1
     on t1.a = driving.a left outer join
     #tmp2 t2
     on t2.id = driving.id and
        t2.a = driving.a
order by t2.id