SQL get only unique combination of two columns

Question

I have table with:

A     B
1     2
2     1

and i trying using sql command to get only one combination

A     B
1     2

how can i do that?

Answer 1

A canonical way in standard SQL is:

select a, b
from t
where a < b
union all
select a, b
from t
where a > b and not exists (select 1 from t t2 where t2.a = t.b and t2.b = t.a);

Note that this assumes no duplicates or equal values. You can easily handle these using select distinct and <= comparisons. In my experience, this problem often arises when there are at most two rows per pair.

This preserves the original values. So, if you start with:

1    2
5    4

You will get that in the result set.

If you don't care about ordering, then many databases support least() / greatest() :

select least(a, b) as a, greatest(a, b) as b
from t
group by least(a, b), greatest(a, b);

You can do the same thing with case expressions. Or, more simply as:

select distinct least(a, b) as a, greatest(a, b) as b
from t;