If your data base supports a full outer join you might get by
with one statement something like:
select nvl(a.username,b.username) from team a full outer join
users b on a.username = b.username
where a.username is null or b.username is null
Otherwise you will need to do something like one of the
following:
(select username from team
minus
select username from users)
union
(select username from users
minus
select username from team)
select a.username from team a left outer join users b on
a.username = b.username
where b.username is null
union
select b.username from users b left outer join team a on
b.username = a.username
where a.username is null