How to combine sql select queries for this situation? - mysql

Question

i'm writing a chat app with php/mysql
i have 3 tables: user, room and room_participant with these structures:
user: id, username
room: id, title
room_participant: room_id, user_id
Now i want to get list of all rooms along with list of all participants in each room.
Until now i just select all rooms from room table and iterate through all rooms and select users information out of each entry, which is very inefficient.
Is there any way to combine all these select into only 1 select query?

Answer 1

Not certain about this without testing, but give it a try:
SELECT
room.*,
user.*
FROM room
JOIN room_participant ON room_id = room_participant.id
JOIN user ON room_participant.user_id = user.id
ORDER BY room.id
To deduplicate rooms, use GROUP_CONCAT()
UPDATE GROUP_CONCAT() modified to return id|username
SELECT
room.id, room.name
GROUP_CONCAT(CONCAT(user.id,'|',user.username)) AS userlist
FROM room
JOIN room_participant ON room_id = room_participant.id
JOIN user ON room_participant.user_id = user.id
GROUP BY room.id, room.name
ORDER BY room.id
With the userlist generated by GROUP_CONCAT as id|name,id|name,id|name you can use PHP explode() to separate them.
// Split the list on the commas
$users = explode(",", $userlist);
$final_users = array();
// Then split each on the `|`
foreach ($users as $user) {
$split_user = explode("|", $user);
// Append each as a new associative array to $final_users
$final_users[] = array('id' => $split_user[0], 'username' => $split_user[1]);
}
// Now you have an array of users as id, username
var_dump($final_users);

Answer 2

You may wish to do two queries, and then match things up in whatever is using MySQL, e.g. PHP.
These two:
SELECT id, title
FROM room;
SELECT rp.room_id, rp.user_id, u.username
FROM room_participant AS rp
INNER JOIN user as u ON rp.user_id = u.id;
Or these two:
SELECT id, username
FROM user;
SELECT rp.room_id, rp.user_id, r.title
FROM room_participant AS rp
INNER JOIN room as r ON rp.user_id = r.id;
Which two queries make sense depends on what you're doing with the info really.
You could go a step further and select all three separately:
SELECT *
FROM room;
SELECT *
FROM user;
SELECT *
FROM room_participant;
Note: It's probably better to state the columns, rather than using '*', just in case in the future new columns are added to the table that you're not really interested in for these queries.
Obviously, you'd then have to match everything up in whatever is using MySQL, e.g. PHP. You could create a list of rooms and users from the selected info, and then match them up with something like:
// Use MySQL to populate $roomList from database, then do...
foreach ($roomList as $room)
{
$id = $room['id'];
$title = $room['title'];
$this->roomList[$id] = new Room($id, $title);
}
// Use MySQL to populate $userList from database, then do...
foreach ($userList as $user)
{
$id = $user['id'];
$username = $user['username'];
$this->userList[$id] = new User($id, $username);
}
// Use MySQL to populate $roomParticipantList from database, then do...
foreach ($roomParticipantList as $roomParticipant)
{
$room = $this->roomList[$roomParticipant['room_id']];
$user = $this->userList[$roomParticipant['user_id']];
// You could do one/both of these, depending on requirements.
$room->enterUser($user);
$user->joinRoom($room);
}