Oracle, Select Distinct columns, with corresponding columns - oracle

Question

I want to select DISTINCT results from the user_id column but I need the corresponding columns as well.
Result set needs to return two role_id that are Distnct user_id and be not an 'Unassigned' status.
The query I am using:
SELECT role_id, user_id, role_code, status_code FROM table where school_id=5 and status_code= 'DRAFT';
This an example of my table:
ROLE_ID USER_ID SCHOOL_ID CAMPUS_ID ROLE_CODE STATUS_CODE
1 4 5 7 Unassigned DRAFT
2 4 5 7 TEST DRAFT
3 4 5 8 TEST DRAFT
4 5 5 9 Unassigned DRAFT
5 5 5 9 TEST DRAFT
6 5 5 10 TEST DRAFT
I have tried to add group by based on user_id but I get an ORA-00979.

Answer 1

You can use ROW_NUMBER() to identify the rows you want. For example:
select *
from (
select t.*,
row_number() over(partition by user_id order by role_id) as rn
from t
where role_code <> 'Unassigned'
) x
where rn = 1

Answer 2

DISTINCT is across the entire set of columns and not for one specific column. Therefore, if you want to get the DISTINCT rows which are not Unassigned you can use:
SELECT DISTINCT
role_id,
user_id,
role_code,
status_code
FROM table
where school_id = 5
and status_code = 'DRAFT'
and role_code != 'Unassigned';
If you want to get a single row for each user_id then you can use GROUP BY and find the minimum role_id:
SELECT MIN(role_id) AS role_id,
user_id,
MIN(role_code ) KEEP (DENSE_RANK FIRST ORDER BY role_id) AS role_code,
MIN(status_code) KEEP (DENSE_RANK FIRST ORDER BY role_id) AS status_code
FROM table
where school_id = 5
and status_code = 'DRAFT'
and role_code != 'Unassigned'
GROUP BY
user_id;