PostgreSQL get aggregated results based on two tables - postgresql

Question

I have two tables,
1. items
2. festival_rents
Sample records:
items
id | name | rent
------------------------
1 | Car | 100
2 | Truck | 150
3 | Van | 200
Sample records:
festival_rents
id | items_id | start_date | end_date | rent
------------------------------------------------
1 | 1 | 2018-07-01 | 2018-07-02 | 200
2 | 1 | 2018-07-04 | 2018-07-06 | 300
3 | 3 | 2018-07-06 | 2018-07-07 | 400
The table items contains list of items with name and rent. Each item in the items table may or may not have festival_rents. The table festival_rents has higher rents for each item for a date range with start_date and end_date. It is possible for a item to have multiple festival_rents with different date ranges. But it's for sure that date ranges for multiple festival_rents belonging to a same item won't collide and all date ranges are isolated.
The query that I'm looking for is, for a given start_date and end_date range, for each item in the items table, calculate the total rent and display each item with it's calculated total rent. The rent calculation for each item should include the festival_rents also, if any of the items has festival_rents falling within the given start_date and end_date.
Expected result:
Input: start_date=2018-07-01 and end_date=2018-07-06
Output:
id | name | total_price
------------------------
1 | Car | 1100 // 1st 2 days festival rent + 1 day normal rent + last 3 days festival rent (2 * 200) + (1 * 100) + (3 * 200)
2 | Truck | 900 // 6 days normal rent (6 * 150)
3 | Van | 1400 // 5 days normal rent + 1 day festival rent (200 * 5) + (400 * 1)

Answer 1

You need a list of days either on a table or create on the fly:
How to get list of dates between two dates in mysql select query
Generating time series between two dates in PostgreSQL
SELECT i.name, SUM (f.rent)
FROM allDays a
JOIN festival_rents f
ON a.day >= f.start_date
AND a.day < f.end_date
JOIN items i
ON f.item_id = i.item_id
WHERE a.day BETWEEN #start_date
AND #end_date
GROUP BY i.name
I assume the end_date is open range. So if you have ranges [A,B) and [B,C) Date B will have the rent from [B,C)