Doctrine from the basics to the more advanced topics. Bear in mind that the slides on their own are not very useful without the video!
https://www.youtube.com/watch?v=DYvK9j-jEcU
username = 'albert' SQL SELECT u.*,g.* FROM users JOIN users_groups ug ON ug.username = u.user_id JOIN groups g ON g.name = ug.group_id WHERE username = 'albert'