Selecting and deleting duplicate rows

This is a question I've seen a few times, and
had to research, so I figured I'd share the
answer.


-------------------------------------------------


drop table test;
--
create table test ( a text, b text );
--  unique values
insert into test values ( 'x', 'y');
insert into test values ( 'x', 'x');
insert into test values ( 'y', 'y' );
insert into test values ( 'y', 'x' );
-- duplicate values
insert into test values ( 'x', 'y');
insert into test values ( 'x', 'x');
insert into test values ( 'y', 'y' );
insert into test values ( 'y', 'x' );
-- one more double duplicate
insert into test values ( 'x', 'y');
--
select oid, a, b from test;
--
-- select duplicate rows
--
select o.oid, o.a, o.b from test owhere exists ( select 'x'                  from test i                where i.a = o.a
                and i.b = o.b                  and i.oid < o.oid            );
 
--
-- delete duplicate rows
--
-- Note: PostgreSQL dosn't support aliases on
--       the table mentioned in the from clause
--       of a delete.
--
delete from test where exists ( select 'x'                  from test i                where i.a = test.a
  and i.b = test.b                  and i.oid < test.oid            );
 
--
-- Let's see if it worked.
--

select oid, a, b from test;

--
-- Delete duplicates with respect to a only, ignoring
-- the value in b.  Note, the first deletion leaves the 
-- first oid with the unique values and removes subsequent
-- ones, in this delete we reverse the direction of the <
-- to save the last oid, and remove the previous ones.
--

delete from test where exists ( select 'x'                  from test i                where i.a = test.a
  and i.oid > test.oid            );
 

--
-- Let's see if it worked.
--

select oid, a, b from test;