How to check if a given data exists in multiple tables (all of which has the same column)?

Question

I have 3 tables, each consisting of a column called username. On the registration part, I need to check that the requested username is new and unique.

I need that si

Answer 1

select 1 
from (
    select username as username from tbl1
    union all
    select username from tbl2
    union all
    select username from tbl3
) a
where username = 'someuser'