this is part of a query result :
all three columns are
user_ids acquired from multiple tables join .
as you see each row has only one user_id and two other user_ids are necessarily
I'm going to group this rows by
user_id regardless of the column it is on.
so i need to have all user_ids in one column (instead of three) to make me enable to perform
group by on it. the ultimate result should be like this: