Bon en fait c'était surtout parce que j'ai fais n'importe quoi dans mes jointures...
la bonne syntaxe :
SELECT users_sessions.id AS users_sessions_id,
users_sessions.user_id AS users_sessions_user_id,
users.lastname AS users_lastname, users.firstname AS
users_firstname
FROM prefix_users_sessions AS users_sessions
JOIN prefix_users AS users ON users.id = users_sessions.user_id
Bon week-end !