ราคาเฉลี่ยสำหรับบทเรียนที่สอนโดย Janessa Sawayn คือเท่าใด?ไม่จำเป็นต้องใช้ตารางทั้งหมด
What is the average price for a lesson taught by Janessa Sawayn?It is not neccessary to use all the tables.
 lessons:  lesson_id [ number]  customer_id [ number]  lesson_status_code [ text]  staff_id [ number]  vehicle_id [ number]  lesson_date [ time]  lesson_time [ text]  price [ number] 
SELECT AVG(price) FROM lessons AS T1 JOIN staff AS T2 ON T1.staff_id = T2.staff_id WHERE T2.first_name = "Janessa" AND T2.last_name = "Sawayn"