sql - 没有重复组合的交叉连接

Question

我知道这个问题与这个问题非常相似:
Symmetric cross join
还有这个:
combinations (not permutations) from cross join in sql

但是如果我们有两个不同的表，比如说 A 和 B:

select A.id,B.id from A cross join B

我想考虑这对 (a,b)等于 (b,a) ?

Answer 1

select A.id aid,B.id bid
from A inner join B on a.id <= b.id
union
select B.id,A.id
from A inner join B on b.id < a.id

如果你想变得更复杂:

select distinct
       case when a.id<=b.id then a.id else b.id end id1,
       case when a.id<=b.id then b.id else a.id end id2
from A cross join B

在我用小 table 进行的不科学的小烘焙中，后者更快。下面是 case写成子查询的表达式。

select distinct
       (select MIN(id) from (select a.id union select b.id)[ ]) id1,
       (select MAX(id) from (select a.id union select b.id)[ ]) id2
from A cross join B