題目：
Table: Prices
+---------------+---------+
| Column Name | Type |
+---------------+---------+
| product_id | int |
| start_date | date |
| end_date | date |
| price | int |
+---------------+---------+
(product_id临燃，start_date募闲，end_date) 是 Prices 表的主鍵年碘。
Prices 表的每一行表示的是某個產(chǎn)品在一段時期內(nèi)的價格忆某。
每個產(chǎn)品的對應(yīng)時間段是不會重疊的，這也意味著同一個產(chǎn)品的價格時段不會出現(xiàn)交叉挣惰。
Table: UnitsSold
+---------------+---------+
| Column Name | Type |
+---------------+---------+
| product_id | int |
| purchase_date | date |
| units | int |
+---------------+---------+
UnitsSold 表沒有主鍵，它可能包含重復項。
UnitsSold 表的每一行表示的是每種產(chǎn)品的出售日期膝宁，單位和產(chǎn)品 id。
編寫SQL查詢以查找每種產(chǎn)品的平均售價根吁。
average_price 應(yīng)該四舍五入到小數(shù)點后兩位员淫。
查詢結(jié)果格式如下例所示：
Prices table:
+------------+------------+------------+--------+
| product_id | start_date | end_date | price |
+------------+------------+------------+--------+
| 1 | 2019-02-17 | 2019-02-28 | 5 |
| 1 | 2019-03-01 | 2019-03-22 | 20 |
| 2 | 2019-02-01 | 2019-02-20 | 15 |
| 2 | 2019-02-21 | 2019-03-31 | 30 |
+------------+------------+------------+--------+
UnitsSold table:
+------------+---------------+-------+
| product_id | purchase_date | units |
+------------+---------------+-------+
| 1 | 2019-02-25 | 100 |
| 1 | 2019-03-01 | 15 |
| 2 | 2019-02-10 | 200 |
| 2 | 2019-03-22 | 30 |
+------------+---------------+-------+
Result table:
+------------+---------------+
| product_id | average_price |
+------------+---------------+
| 1 | 6.96 |
| 2 | 16.96 |
+------------+---------------+
平均售價 = 產(chǎn)品總價 / 銷售的產(chǎn)品數(shù)量。
產(chǎn)品 1 的平均售價 = ((100 * 5)+(15 * 20) )/ 115 = 6.96
產(chǎn)品 2 的平均售價 = ((200 * 15)+(30 * 30) )/ 230 = 16.96
mysql:
SELECT
product_id,
Round(SUM(sales) / SUM(units), 2) AS average_price
FROM (
SELECT
Prices.product_id AS product_id,
Prices.price * UnitsSold.units AS sales,
UnitsSold.units AS units
FROM Prices
JOIN UnitsSold ON Prices.product_id = UnitsSold.product_id
WHERE UnitsSold.purchase_date BETWEEN Prices.start_date AND Prices.end_date
) T
GROUP BY product_id
