sql为每月选择一条记录，并记录该月记录的总和

I have a database that has an "appointments table" and "services table". Each appt has a service and each service has a price. What I would like is one query, that will always return 12 rows (one row for each month) and contain a sum of the months appts (based on it's service id). So far I have:I have a database that has an "appointments tab