Something like this, if you are on PostgreSQL 9.1
with test_insert as (
insert into foo (id, col1, col2)
select 42, 'Foo', 'Bar'
where not exists (select * from foo where id = 42)
returning foo.id, foo.col1, foo.col2
)
select id, col1, col2
from test_insert
union
select id, col1, col2
from foo
where id = 42;
It's a bit longish and you need to repeat the id to test for several times, but I can't think of a different solution that involves a single SQL statement.
If a row with id=42
exists, the writeable CTE will not insert anything and thus the existing row will be returned by the second union part.
When testing this I actually thought the new row would be returned twice (therefor a union
not a union all
) but it turns out that the result of the second select statement is actually evaluated before the whole statement is run and it does not see the newly inserted row. So in case a new row is inserted, it will be taken from the "returning" part.