Problem: 1251. 平均售价
👨🏫 参考题解（题目数据增强，代码只能过90%的点）
🍻 AC code
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
WHERE u.purchase_date BETWEEN p.start_date AND p.end_date or u.product_id is null
GROUP BY p.product_id
博览会，8月20-22日，国家会展中心（上海）)
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