EXISTS clause in SQL

I have following requirement: main_set table has all the ids, user_input table has all user entered ids.

I want result of ids that exist in user_input table. If nothing in user_input table then I want all the ids from main_input table.

Here's what I have so far:

create table main_set as
select 1 id from dual
union all
select 2 from dual
union all
select 3 from dual

create table user_input as
select 1 id from dual
union all
select 2 from dual

select *
  from main_set ms
  where exists (select null
                        from user_input ui
                       where ui.id = ms.id)

select id from user_input
union all
select id from main_set where not exists(select * from user_input);

--edit based on your comment

select *
from main_set
where id in(select id from user_input) or not exists(select * from user_input)

or, seeing as you mention performance, perhaps you should try a join as an alternative:

select main_set.*
from main_set left outer join user_input on(user_input.id=main_set.id)
where user_input.id is not null or not exists(select * from user_input)

StackExchange Database Administrators Q#4900, answer score: 3