没有重复组合的交叉连接 [英] Cross Join without duplicate combinations

问题描述

我知道这个问题与这个问题非常相似:对称交叉连接还有这个:sql 中交叉连接的组合(不是排列)>

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 (a,b) 等于 (b,a) ?

解决方案

select A.id aid,B.id bid从 A 内连接 B 上 a.id <= b.id联盟选择 B.id,A.id从 A 内部连接 ​​B 上 b.id <援助

如果你想变得更老练:

选择不同的当 a.id<=b.id 然后 a.id else b.id end id1,当 a.id<=b.id then b.id else a.id end id2从 A 交叉连接 B

在我用小桌子进行的不科学的小烘焙中，后者更快.以及下面，写为子查询的 case 表达式.

选择不同的(select MIN(id) from (select a.id union select b.id)[ ]) id1,(select MAX(id) from (select a.id union select b.id)[ ]) id2从 A 交叉连接 B

I know this question is very similar to this one: Symmetric cross join and this one too: combinations (not permutations) from cross join in sql

But what about if we have two different tables, say A and B:

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

and I want to consider the pair (a,b) equal to (b,a) ?

解决方案

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

If you wanted to be more sophisticated:

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

In my little unscientific bake off with tiny tables, the latter was faster. And below, the case expressions written as subqueries.

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

这篇关于没有重复组合的交叉连接的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！