select id from users where id=123456
union all
select id from users_archived where id = 123456;

SELECT GREATEST(@found := -1, id) AS id, 'users' AS which_tbl
FROM users WHERE id  = 1
UNION ALL
    SELECT id, 'users_archived'
    FROM users_archived WHERE id = 1 and @found IS NULL
UNION ALL
    SELECT 1, 'reset' FROM DUAL WHERE (@found := NULL) IS NOT NLL;