1251. Average Selling Price

Table: Prices

Column Name	Type
product_id	int
start_date	date
end_date	date
price	int

(product_id, start_date, end_date) is the primary key (combination of columns with unique values) for this table.
Each row of this table indicates the price of the product_id in the period from start_date to end_date.
For each product_id there will be no two overlapping periods. That means there will be no two intersecting periods for the same product_id.

Table: UnitsSold

Column Name	Type
product_id	int
purchase_date	date
units	int

This table may contain duplicate rows.
Each row of this table indicates the date, units, and product_id of each product sold.

Write a solution to find the average selling price for each product. average_price should be rounded to 2 decimal places.

Return the result table in any order.

The result format is in the following example.

Example 1:

Input:
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

Output:

product_id	average_price
1	6.96
2	16.96

Explanation:
Average selling price = Total Price of Product / Number of products sold.
Average selling price for product 1 = ((100 5) + (15 20)) / 115 = 6.96
Average selling price for product 2 = ((200 15) + (30 30)) / 230 = 16.96

SELECT p.product_id , IFNULL(ROUND(SUM(p.price*u.units)/SUM(u.units),2),0) AS average_price
FROM Prices p
LEFT JOIN UnitsSold 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

學習點:
1.IFNULL()
2.日期查看區間 BETWEEN