sql - sql中对的分组

Question

我想在 SQL 中将 (a,b) 和 (b,a) 归为一组 (a,b)

例如下面的集合

SELECT 'a' AS Col1, 'b' AS Col2
UNION ALL
SELECT 'b', 'a'
UNION ALL
SELECT 'c', 'd'
UNION ALL
SELECT 'a', 'c'
UNION ALL
SELECT 'a', 'd'
UNION ALL
SELECT 'b', 'c'
UNION ALL
SELECT 'd', 'a'

应该屈服

Col1 | Col2
a       b
c       d
a       c
a       d
b       c

Answer 1

按按字母顺序选择对的 case 语句分组:

select case when col1 < col2 then col1 else col2 end as col1,
case when col1 < col2 then col2 else col1 end as col2
from (
    select 'a' as col1, 'b' as col2
    union all
    select 'b', 'a'
    union all
    select 'c', 'd'
    union all
    select 'a', 'c'
    union all
    select 'a', 'd'
    union all
    select 'b', 'c'
    union all
    select 'd', 'a'
) t group by case when col1 < col2 then col1 else col2 end,
case when col1 < col2 then col2 else col1 end

http://sqlfiddle.com/#!3/9eecb7db59d16c80417c72d1/6977

如果您只是想要唯一值(而不是聚合分组)，那么您可以使用 distinct 而不是 group by

select distinct case when col1 < col2 then col1 else col2 end as col1,
case when col1 < col2 then col2 else col1 end as col2
from (
    select 'a' as col1, 'b' as col2
    union all
    select 'b', 'a'
    union all
    select 'c', 'd'
    union all
    select 'a', 'c'
    union all
    select 'a', 'd'
    union all
    select 'b', 'c'
    union all
    select 'd', 'a'
) t