Join sub queries in PL-SQL

I know this is really basic, but I'm stuck. I'm unable to select specific columns from a join of sub queries. For example -

This works -

select * from
(subquery) x
left join
(subquery) y
on x.id = y.id

This also works -

select x.id from
(subquery) x
left join
(subquery) y
on x.id = y.id

This works as well -

select y.id from
(subquery) x
left join
(subquery) y
on x.id = y.id

This does NOT work -

select x.id, y.id from
(subquery) x
left join
(subquery) y
on x.id = y.id

What am I missing in the last query? Appreciate any help. Thanks!