SQL - get from table and join with same table - sql

Question

I have two tables, ChatRoom and ChatRoomMap, I want to get a list of chatrooms a user belongs to, along with all the other users in each chatroom.
// this contains a map of user to chatroom, listing which user is in what room
CREATE TABLE ChatRoomMap
(
user_id bigint NOT NULL,
chatroom_id text NOT NULL,
CONSTRAINT uniq UNIQUE (userid, roomid)
)
// sample values
==========================
| user_id | chatroom_id |
| 1 | 7 |
| 1 | blue |
| 7 | red |
==========================
And
CREATE TABLE ChatRoom
(
id text NOT NULL,
admin bigint,
name text,
created timestamp without time zone NOT NULL DEFAULT now(),
CONSTRAINT uniqid UNIQUE (id)
)
// sample values
======================================================
| id | admin | name | timestamp |
| blue | 7 | blue room | now() |
| red | 2 | red | now() |
| 7 | 11 | mine | now() |
======================================================
To get a list of rooms a user is in, I can do:
SELECT DISTINCT ON (id) id, userid, name, admin
FROM ChatRoomMap, ChatRoom WHERE ChatRoomMap.user_id = $1 AND ChatRoomMap.chatroom_id = ChatRoom.id
This will get me a distinct list of chat rooms a user is in.
I would like to get the distinct list of rooms along with all the users in each room (concatenation of all as a separate column), how can this be done?
Example result:
=======================================================
| user_id | chatroom_id | name | admin | other_users |
| 10 | 7 | One | 1 | 1, 2, 3, 8 |
| 10 | 4 | AAA | 10 | 7, 11, 15 |
=======================================================

Answer 1

First up, use proper joins - the explicit join syntax was introduced to the SQL92 standard and the major vendors implemented it in the early 2000's (and it's the only way to achieve an outer join).
Try this:
SELECT DISTINCT id, crm2.user_id, name, admin,
FROM ChatRoomMap crm1
JOIN ChatRoom ON crm1.chatroom_id = ChatRoom.id
LEFT JOIN ChatRoomMap crm2 ON crm2.chatroom_id = crm1.chatroom_id
AND crm2.user_id != crm1.user_id -- only other users
WHERE crm1.user_id = $1
The LEFT JOIN is needed in case there are no other users in the room it will still list the room (with a null for other user id).