Postgres id to name mapping in an array while creating CSV file - postgresql

Question

I have a table with id to group name mapping.
1. GroupA
2. GroupB
3. GroupC
.
.
.
15 GroupO
And I have user table with userId to group ID mapping, group ID is defined as array in user table
User1 {1,5,7}
User2 {2,5,9}
User3 {3,5,11,15}
.
.
.
I want to combine to table in such a way to retrieve userID and groupName mapping in CSV file.
for example: User1 {GroupA, GroupE, GroupG}
Essentially group ID should get replace by group name while creating CSV file.

Answer 1

Setup:
create table mapping(id int, group_name text);
insert into mapping
select i, format('Group%s', chr(i+ 64))
from generate_series(1, 15) i;
create table users (user_name text, user_ids int[]);
insert into users values
('User1', '{1,5,7}'),
('User2', '{2,5,9}'),
('User3', '{3,5,11,15}');
Step by step (to understand the query, see SqlFiddle):
Use unnest() to list all single user_id in a row:
select user_name, unnest(user_ids) user_id
from users
Replace user_id with group_name by joining to mapping:
select user_name, group_name
from (
select user_name, unnest(user_ids) id
from users
) u
join mapping m on m.id = u.id
Aggregate group_name into array for user_name:
select user_name, array_agg(group_name)
from (
select user_name, group_name
from (
select user_name, unnest(user_ids) id
from users
) u
join mapping m on m.id = u.id
) m
group by 1
Use the last query in copy command:
copy (
select user_name, array_agg(group_name)
from (
select user_name, group_name
from (
select user_name, unnest(user_ids) id
from users
) u
join mapping m on m.id = u.id
) m
group by 1
)
to 'c:/data/example.txt' (format csv)

Answer 2

Say you have two tables in this form:
Table groups
Column | Type
-----------+---------
groupname | text
groupid | integer
Table users
Column | Type
----------+----------
username | text
groupids | integer[] <-- group ids as inserted in table groups
You can query the users replacing the group id with group names with this code:
WITH users_subquery AS (select username,unnest (groupids) AS groupid FROM users)
SELECT username,array_agg(groupname) AS groups
FROM users_subquery JOIN groups ON users_subquery.groupid = groups.groupid
GROUP BY username
If you need the groups as string (useful for the csv export), surround the query with a array_to_string statement:
SELECT username, array_to_string(groups,',') FROM
(
WITH users_subquery AS (select username,unnest (groupids) AS groupid FROM users)
SELECT username,array_agg(groupname) AS groups
FROM users_subquery JOIN groups ON users_subquery.groupid = groups.groupid
GROUP BY username
) as foo;
Result:
username | groups
----------+-----------------
user1 | group1,group2
user2 | group2,group3