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);