Question

I am building a mysql based chat application.

My database schema has the following tables,

       Users               Messages
   =================   =================
        id                 id
        screen_name        message
                           from
                           to
                           timestamp

The from and to fields on the messages table contain the id’s of the users that sent and received each message.

I am trying to display all messages between a user ($id) and one of their friends ($friend). My query is the following:

$query = "SELECT messages.* , users.screen_name FROM users CROSS JOIN messages ";
$query .= "ON ( messages.to = $id AND messages.from = $friend ) ";
$query .= "OR ( messages.to = $friend AND messages.from = $id )";

The problem is that every message is twice in the result table.

I tried using DISTINCT but it either doesn’t work in this scenario or I used it wrong.

What should my query be in order to have each message between the two users only once?

Answer 1

Something like this should do the trick:

SELECT
  messages.*,
  users_from.screen_name AS from_screen_name,
  users_to.screen_name AS to_screen_name
FROM
  messages
    JOIN users AS users_from ON messages.from = users_from.id
    JOIN users AS users_to ON messages.to = users_to.id
WHERE
  (messages.to = $id AND messages.from = $friend)
  OR ( messages.to = $friend AND messages.from = $id)

What this does is joing the “users” table twice, once on the “to” column and the second time on the “from” column.