(SQL)同一列的多行字符串数据怎么转换成同一列的一行数据？

            同一列的多行字符串数据用SQL怎么转换成同一列的一行数据？

比如：我用select department,userName from users从表中查询出如下数据

department | userName

---------------   --------------

it                   it1

it                   it2

it                   it3

ur                  ur1

ur                  ur2

我能不能用什么SQL对department进行分组然后变成如下的结果呢？

department | userName

---------------   --------------

it                   it1_it2_it3

ur                  ur1_ur2

似乎行列转换中只能对那些采用聚合函数的才能转换，这个东东就不知道在SQL中怎么办？谢谢！

drop table users
/
create table users
(department varchar2(20),
 userName  varchar2(20)
)
/
insert into users
values('it','it1');
insert into users
values('it','it2');
insert into users
values('it','it3');
insert into users
values('ur','ur1');
insert into users
values('ur','ur2');
commit
/
select max(count(*))
from users
group by department;
--
select department,
       decode(substr(usernames, -1),
              '_',
              substr(usernames, 1, length(usernames) - 1),
              usernames)
  from (select department,
               max(decode(rn, 1, userName, null)) || '_' ||
               max(decode(rn, 2, userName, null)) || '_' ||
               max(decode(rn, 3, userName, null)) usernames
          from (select department,
                       userName,
                       row_number() over(partition by department order by userName) rn
                  from users)
         group by department);
--