Build MySql queries

I have 3 tables ## Users `user_id` | `subscription_date` --:|:-- 1 | 2015-07-10 2 | 2015-07-15 3 | 2015-07-17 ## Courses `course_id` | `name` 1 | Course A 2 | Course B ## Orders `order_id`  | `user_id` | `purchased_course_id` | `purchased_other_thing1` | `order_status` --:|--:|--:|--:|--: 1 | 2 | 0 | 1 | paid 2 | 2 | 1 | 0 | paid 3 | 3 | 0 | 1 | failed 4 | 3 | 2 | 1 | paid For each user subscribed, and for each courses, I want to know when the user has not paid for the course and for how long since he subscribed. (today is 2015-07-20) Result `user_id` | `course_id` | `unpaid_days` 1 | 1 | 10 (user 1 has never paid course 2) 1 | 2 | 10 (user 1 has never paid course 1) 2 | 1 | NULL (user 2 has paid, null ok 0 is ok) 2 | 2 | 5 (user 2 has not paid course 2) 3 | 1 | 3 (user 3 has not paid course 1) 3 | 2 | NULL (user 3 has paid course 2) I have this one for now, and maybe others.