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Formatted question description: https://leetcode.ca/all/1251.html

# 1251. Average Selling Price

## Level

Easy

## Description

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 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 |
+---------------+---------+
There is no primary key for this table, it may contain duplicates.
Each row of this table indicates the date, units and product_id of each product sold.
```

Write an SQL query to find the average selling price for each product.

`average_price`

should be **rounded to 2 decimal places**.

The query result format is in the following example:

```
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 |
+------------+---------------+
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
```

## Solution

To calculate the average selling price for each product, the total price of the product and the total units of the product need to be obtained.

To calculate the total price, calculate the sum of price * units. To calculate the total units, calculate the sum of units.

For each product, obtain the price according to the purchase date.

Use `round`

to round the result to 2 decimal places and use `ifnull`

to avoid dividing by 0.

```
# Write your MySQL query statement below
select product_id, ifnull(round(sum(prices_sum) / sum(units), 2), 0) as average_price
from (
select p.product_id as product_id, units, price * units as prices_sum
from Prices p left join UnitsSold u
on p.product_id = u.product_id and purchase_date between start_date and end_date
) as temp
group by product_id;
```