Question

i have a PostgreSQL table describing lines between two points. It contains two columns, A and B, integers representing the id of a point (described in another table).

But each line is duplicated in the table, as the line going from A to B is the same that the line going from B to A.

I’d like to remove the duplicates, but i can’t find an aggregate function working on two columns, to regroup AB and BA lines then to remove one.

Thanks 🙂

Answer 1

Identifying the duplicates:

select least(a,b), greatest(a,b), count(*)
from the_table
group by least(a,b), greatest(a,b)
having count(*) > 1

I think you should be able to delete one of the pairs using:

delete from the_table
where (least(a,b), greatest(a,b)) in (
                select least(a,b), greatest(a,b)
                from the_table
                group by least(a,b), greatest(a,b)
                having count(*) > 1);

(Not tested!)