SQL problem - Average Selling Price

Problem

My Query

SELECT
    p.product_id,
    ROUND(IFNULL(SUM(units * price)/SUM(units), 0),2) as average_price
FROM
    Prices as p
LEFT JOIN
    UnitsSold as u
    ON
        p.product_id = u.product_id AND (u.purchase_date BETWEEN p.start_date AND p.end_date)
GROUP BY
    p.product_id