CREATE TABLE table_name_49 (round INTEGER, position VARCHAR, overall VARCHAR)
SELECT AVG(round) FROM table_name_49 WHERE position = "defensive back" AND overall < 199
รอบเฉลี่ยคือเท่าไรเมื่อมีกองหลังและรวมน้อยกว่า 199?
What is the average round when there is a defensive back and an overall smaller than 199?