SQL & PL/SQL :: How To Bypass Putting Select Inside Count Function In Query

I have 2 tables, ASSIGNMENT and RESEARCH_PAPER. For each research paper, I need to find out :

1. The number of assignments created from it (after a given constant assign date)

2. The number of assignments created from it that have been approved.

3. The number of unique users who have either created or approved an assignment from it

Test data :

create table research_paper (id int, name varchar2(100));
create table assignment (id int, r_paper_id int, assigner_id int, assignee_id int,
approver_id int, assign_date timestamp, approved_yn varchar2(10));
insert into research_paper values (1, 'A');
insert into research_paper values (2, 'B');

[code]....

Assignment :

id r_paper_id assigner_id assignee_id approver_id assign_date approved_yn
-----------------------------------------------------------------------------------------------------------
11 100 200 100 23-10-12 12:00:00.000000000 AMY
22 200 100 200 22-10-12 12:00:00.000000000 AMN
32 100 200 101 24-10-12 12:00:00.000000000 AMY

[code]....

Research_paper:

id name
----------
1A
2B

Expected result :

r_paper_id created approved unique_users
-----------------------------------------------
1 3 2 4
2 3 2 3

I wrote the following query for that :

SELECT rp.id r_paper_id,
COUNT(*) created,
COUNT(
CASE
WHEN a.approved_yn = 'Y'

[code]....

But it fails, saying that 'single-row subquery returns more than one row' when I introduce the 'unique_users' clause. The remaining fields of the output are correct.