Question

I have the following tables :

User_Group

id  group_id  group_type
------------------------
1   100       A
1   100       B
2   101       B
2   102       A

Group_A

id   name
---------
100  A
101  B
102  C

Group_B

id   name
---------
100  D
101  E
102  F

I want the group names of all users (using array.agg()). We have to get the group name from group A if the user’s group type = A and from group B if the user’s group type = B. The result should be :

userid  groups
--------------
1       A,D
2       E,C

I have created a fiddle for this, and given a solution using union of 2 separate queries. Can it be done without the union, something in which I can decide on which table to pick the group name from with a single join of user_groups, group_A and group_B ?

Answer 1

select ug.id, array_agg(
    case ug.group_type 
        when 'A' then g_a.name 
        when 'B' then g_b.name 
        else 'N/A' 
    end)
from user_groups ug
left outer join group_A g_a on ug.group_id = g_a.id
left outer join group_B g_b on ug.group_id = g_b.id
group by ug.id

SQL Fiddle Example