Join a series of values in different rows into one

Question

I need to do an ETL job and I have a table that looks like this

user_name | role
----------+------
User A    | Admin
User A    | System
User B    | Editor
User B    | Power User
User B    | System

I want to transform that into

user_name | role
----------+------
User A    | Admin, System
User B    | Editor, Power User, System

I thought about creating temp tables and using row_number() over (partition by user_name) but it started to get messy. Is there a better way to get that output?

Answer 1

Use the aggregate function string_agg().

SELECT user_name, string_agg("role", ', ') AS roles
FROM   tbl
GROUP  BY 1
ORDER  BY 1;

If you want the list in alphabetical order:

string_agg("role", ', ' ORDER BY "role")

Answer 2

SELECT user_name, 
 array_to_string(array(
        select role
        from my_table
        where user_name =m.user_name
        order by user_name
        ), ', '
    ) combined_rows
from my_table m

group by 1

Link to SQL Fiddle