Count days of daterange in daterange - mysql

Question

I want to count how many days of a start and end-date are inside another start and end-date. Basically: How many days of the date 01.06.2020 - 06.06.2020 are inside the date 03.06.2020 - 31.12.2020. In this case the answer would be 4. The 3rd, 4th, 5th and 6th of June 2020.
I know that I can use TIMESTAMPDIFF to do calculations between 2 dates, but I can't wrap my head around a simple solution to do it with 2 dates inside 2 dates.
I can't believe nobody ever asked that question, but I can't find any solution to this.
Simple table:
+-------------+--------------+
| Start_date | End_date |
+-------------+--------------+
| 2020-06-03 | 2020-12-31 |
| 2014-09-08 | 2015-09-07 |
| 2015-01-15 | 2015-02-01 |
+-------------+--------------+
I look for something along the lines of:
SELECT * FROM available_dates WHERE TIMESTAMPDIFF(DAY,'2020-06-01','2020-06-06', Start_date, End_date) > 5

Answer 1

You can do this by taking the intersection of the two ranges using GREATEST on the start dates and LEAST on the end dates. Intersecting ranges will return a positive number, non-intersecting a negative one. So we use GREATEST again to zero out negative values to get the overlap. For example:
CREATE TABLE test (
`Start_date` DATE,
`End_date` DATE
);
INSERT INTO test
(`Start_date`, `End_date`)
VALUES
('2020-06-03', '2020-12-31'),
('2014-09-08', '2015-09-07'),
('2015-01-15', '2015-02-01');
SET #start = '2015-01-20';
SET #end = '2015=02-04';
SELECT GREATEST(TIMESTAMPDIFF(DAY, GREATEST(Start_Date, #start), LEAST(End_Date, #end))+1, 0) AS overlap
FROM test;
Output:
overlap
0
16
13
Demo on dbfiddle