MySQL JOIN vrací pole NULL

Nejste si jisti svými daty a schématem, ale pokud je up.profile_value jako "123", můžete zkusit toto:

SELECT u.name AS _user_name, s.name AS _school_name
FROM fwg_files AS f
LEFT JOIN users AS u ON u.id = f.user_id
LEFT JOIN user_profiles AS up ON up.user_id = u.id
LEFT JOIN school AS s ON CONCAT('"', s.id, '"') = up.profile_value