How to find interchanged rows in a database table.

Question

Hi All,
Today I encountered a strange problem and as I'm not very good at SQL I'm unable to fix it. So please help me out in fixing this issue.
The issue goes like this. I had a table with two columns say x and y. It should allow only values of one pair only i.e, if a,b is entered b,a cannot be entered. I had already some data in the table and would like to identify such columns where data got interchanged. So can anyone help me out in fixing it.
Any help is appreciated.
Thanks and Regards,
V.Venkateswara Rao.

Answer 1

create table no_cons ( x number, y number );
insert into no_cons values ( 1, 2 );
insert into no_cons values ( 2, 1 );
create table has_cons (x number, y number );
create unique index has_cons_uq on has_cons ( to_char(least(x,y)) ||'.'|| to_char(greatest(x,y)) );
begin
  for i in ( select * from no_cons ) loop
    begin
    insert into has_cons values ( i.x,i.y );
    exception
      when dup_val_on_index then
        null;
    end;
  end loop;
end;
select * from no_cons;
     X          Y
     1          2
     2          1
select * from has_cons;
     X          Y
     1          2
select * from no_cons
minus
select * from has_cons;
     X          Y
     2          1