create table `users` (`id` integer not null primary key auto_increment, `username` text not null);

insert into `users` (`username`) values ('a'), ('b'), ('c');

select * from `users` where (`id` IN (

    (select `id` from `users` where `username` = 'a') 

    UNION 

    (select `id` from `users` where `username` = 'c')));

ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'UNION (select `id` from `users` where `username` = 'c')))' at line 1

select * from `users` where (`id` IN (

    select `id` from `users` where `username` = 'a' 

    UNION 

    (select `id` from `users` where `username` = 'c')));

+----+----------+

| id | username |

+----+----------+

|  1 | a        |

|  3 | c        |

+----+----------+