MySQL 8.0

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CREATE TABLE test ( start DATETIME, length INT );
INSERT INTO test VALUES
('2020-10-01 15:05:00',        4),
('2020-10-01 15:35:00',        3),
('2020-10-01 16:10:00',        8),
('2020-10-01 16:25:00',        6);
SELECT * FROM test;


start
length


2020-10-01 15:05:00
4

2020-10-01 15:35:00
3

2020-10-01 16:10:00
8

2020-10-01 16:25:00
6


SET @chunk := '00:15:00';
WITH RECURSIVE
cte1 AS ( SELECT FROM_UNIXTIME((UNIX_TIMESTAMP(MIN(start)) DIV 3600) * 3600) start,
                 TIMESTAMP(FROM_UNIXTIME((UNIX_TIMESTAMP(MIN(start)) DIV 3600) * 3600), @chunk) finish
          FROM test
        UNION ALL
          SELECT TIMESTAMP(start, @chunk), TIMESTAMP(TIMESTAMP(start, @chunk), @chunk)
          FROM cte1
          WHERE start < ( SELECT MAX(start + INTERVAL length HOUR)
                          FROM test ) )
,
cte2 AS ( SELECT start, start + INTERVAL length HOUR finish
          FROM test )
SELECT cte1.start, 
       cte1.finish,
       SUM(TIMESTAMPDIFF(MINUTE, GREATEST(cte1.start, cte2.start), LEAST(cte1.finish, cte2.finish))) total,
       GROUP_CONCAT(cte2.start) starts
FROM cte1
JOIN cte2 ON cte1.start <= cte2.finish AND cte2.start <= cte1.finish
GROUP BY cte1.start, 
         cte1.finish;


start
finish
total
starts


2020-10-01 15:00:00
2020-10-01 15:15:00.000000
10
2020-10-01 15:05:00

2020-10-01 15:15:00
2020-10-01 15:30:00.000000
15
2020-10-01 15:05:00

2020-10-01 15:30:00
2020-10-01 15:45:00.000000
25
2020-10-01 15:35:00,2020-10-01 15:05:00

2020-10-01 15:45:00
2020-10-01 16:00:00.000000
30
2020-10-01 15:35:00,2020-10-01 15:05:00

2020-10-01 16:00:00
2020-10-01 16:15:00.000000
35
2020-10-01 16:10:00,2020-10-01 15:35:00,2020-10-01 15:05:00

2020-10-01 16:15:00
2020-10-01 16:30:00.000000
50
2020-10-01 16:25:00,2020-10-01 16:10:00,2020-10-01 15:35:00,2020-10-01 15:05:00

2020-10-01 16:30:00
2020-10-01 16:45:00.000000
60
2020-10-01 15:35:00,2020-10-01 16:10:00,2020-10-01 16:25:00,2020-10-01 15:05:00

2020-10-01 16:45:00
2020-10-01 17:00:00.000000
60
2020-10-01 16:25:00,2020-10-01 16:10:00,2020-10-01 15:05:00,2020-10-01 15:35:00

2020-10-01 17:00:00
2020-10-01 17:15:00.000000
60
2020-10-01 15:35:00,2020-10-01 16:10:00,2020-10-01 16:25:00,2020-10-01 15:05:00

2020-10-01 17:15:00
2020-10-01 17:30:00.000000
60
2020-10-01 16:25:00,2020-10-01 16:10:00,2020-10-01 15:05:00,2020-10-01 15:35:00

2020-10-01 17:30:00
2020-10-01 17:45:00.000000
60
2020-10-01 15:05:00,2020-10-01 16:10:00,2020-10-01 16:25:00,2020-10-01 15:35:00

2020-10-01 17:45:00
2020-10-01 18:00:00.000000
60
2020-10-01 16:25:00,2020-10-01 16:10:00,2020-10-01 15:05:00,2020-10-01 15:35:00

2020-10-01 18:00:00
2020-10-01 18:15:00.000000
60
2020-10-01 16:25:00,2020-10-01 16:10:00,2020-10-01 15:05:00,2020-10-01 15:35:00

2020-10-01 18:15:00
2020-10-01 18:30:00.000000
60
2020-10-01 16:10:00,2020-10-01 16:25:00,2020-10-01 15:05:00,2020-10-01 15:35:00

2020-10-01 18:30:00
2020-10-01 18:45:00.000000
50
2020-10-01 16:10:00,2020-10-01 15:05:00,2020-10-01 16:25:00,2020-10-01 15:35:00

2020-10-01 18:45:00
2020-10-01 19:00:00.000000
45
2020-10-01 16:25:00,2020-10-01 15:05:00,2020-10-01 16:10:00

2020-10-01 19:00:00
2020-10-01 19:15:00.000000
35
2020-10-01 16:10:00,2020-10-01 15:05:00,2020-10-01 16:25:00

2020-10-01 19:15:00
2020-10-01 19:30:00.000000
30
2020-10-01 16:10:00,2020-10-01 16:25:00

2020-10-01 19:30:00
2020-10-01 19:45:00.000000
30
2020-10-01 16:10:00,2020-10-01 16:25:00

2020-10-01 19:45:00
2020-10-01 20:00:00.000000
30
2020-10-01 16:10:00,2020-10-01 16:25:00

2020-10-01 20:00:00
2020-10-01 20:15:00.000000
30
2020-10-01 16:10:00,2020-10-01 16:25:00

2020-10-01 20:15:00
2020-10-01 20:30:00.000000
30
2020-10-01 16:10:00,2020-10-01 16:25:00

2020-10-01 20:30:00
2020-10-01 20:45:00.000000
30
2020-10-01 16:10:00,2020-10-01 16:25:00

2020-10-01 20:45:00
2020-10-01 21:00:00.000000
30
2020-10-01 16:10:00,2020-10-01 16:25:00

2020-10-01 21:00:00
2020-10-01 21:15:00.000000
30
2020-10-01 16:10:00,2020-10-01 16:25:00

2020-10-01 21:15:00
2020-10-01 21:30:00.000000
30
2020-10-01 16:25:00,2020-10-01 16:10:00

2020-10-01 21:30:00
2020-10-01 21:45:00.000000
30
2020-10-01 16:25:00,2020-10-01 16:10:00

2020-10-01 21:45:00
2020-10-01 22:00:00.000000
30
2020-10-01 16:25:00,2020-10-01 16:10:00

2020-10-01 22:00:00
2020-10-01 22:15:00.000000
30
2020-10-01 16:25:00,2020-10-01 16:10:00

2020-10-01 22:15:00
2020-10-01 22:30:00.000000
25
2020-10-01 16:10:00,2020-10-01 16:25:00

2020-10-01 22:30:00
2020-10-01 22:45:00.000000
15
2020-10-01 16:10:00

2020-10-01 22:45:00
2020-10-01 23:00:00.000000
15
2020-10-01 16:10:00

2020-10-01 23:00:00
2020-10-01 23:15:00.000000
15
2020-10-01 16:10:00

2020-10-01 23:15:00
2020-10-01 23:30:00.000000
15
2020-10-01 16:10:00

2020-10-01 23:30:00
2020-10-01 23:45:00.000000
15
2020-10-01 16:10:00

2020-10-01 23:45:00
2020-10-02 00:00:00.000000
15
2020-10-01 16:10:00

2020-10-02 00:00:00
2020-10-02 00:15:00.000000
10
2020-10-01 16:10:00

start	length
2020-10-01 15:05:00	4
2020-10-01 15:35:00	3
2020-10-01 16:10:00	8
2020-10-01 16:25:00	6

start	finish	total	starts
2020-10-01 15:00:00	2020-10-01 15:15:00.000000	10	2020-10-01 15:05:00
2020-10-01 15:15:00	2020-10-01 15:30:00.000000	15	2020-10-01 15:05:00
2020-10-01 15:30:00	2020-10-01 15:45:00.000000	25	2020-10-01 15:35:00,2020-10-01 15:05:00
2020-10-01 15:45:00	2020-10-01 16:00:00.000000	30	2020-10-01 15:35:00,2020-10-01 15:05:00
2020-10-01 16:00:00	2020-10-01 16:15:00.000000	35	2020-10-01 16:10:00,2020-10-01 15:35:00,2020-10-01 15:05:00
2020-10-01 16:15:00	2020-10-01 16:30:00.000000	50	2020-10-01 16:25:00,2020-10-01 16:10:00,2020-10-01 15:35:00,2020-10-01 15:05:00
2020-10-01 16:30:00	2020-10-01 16:45:00.000000	60	2020-10-01 15:35:00,2020-10-01 16:10:00,2020-10-01 16:25:00,2020-10-01 15:05:00
2020-10-01 16:45:00	2020-10-01 17:00:00.000000	60	2020-10-01 16:25:00,2020-10-01 16:10:00,2020-10-01 15:05:00,2020-10-01 15:35:00
2020-10-01 17:00:00	2020-10-01 17:15:00.000000	60	2020-10-01 15:35:00,2020-10-01 16:10:00,2020-10-01 16:25:00,2020-10-01 15:05:00
2020-10-01 17:15:00	2020-10-01 17:30:00.000000	60	2020-10-01 16:25:00,2020-10-01 16:10:00,2020-10-01 15:05:00,2020-10-01 15:35:00
2020-10-01 17:30:00	2020-10-01 17:45:00.000000	60	2020-10-01 15:05:00,2020-10-01 16:10:00,2020-10-01 16:25:00,2020-10-01 15:35:00
2020-10-01 17:45:00	2020-10-01 18:00:00.000000	60	2020-10-01 16:25:00,2020-10-01 16:10:00,2020-10-01 15:05:00,2020-10-01 15:35:00
2020-10-01 18:00:00	2020-10-01 18:15:00.000000	60	2020-10-01 16:25:00,2020-10-01 16:10:00,2020-10-01 15:05:00,2020-10-01 15:35:00
2020-10-01 18:15:00	2020-10-01 18:30:00.000000	60	2020-10-01 16:10:00,2020-10-01 16:25:00,2020-10-01 15:05:00,2020-10-01 15:35:00
2020-10-01 18:30:00	2020-10-01 18:45:00.000000	50	2020-10-01 16:10:00,2020-10-01 15:05:00,2020-10-01 16:25:00,2020-10-01 15:35:00
2020-10-01 18:45:00	2020-10-01 19:00:00.000000	45	2020-10-01 16:25:00,2020-10-01 15:05:00,2020-10-01 16:10:00
2020-10-01 19:00:00	2020-10-01 19:15:00.000000	35	2020-10-01 16:10:00,2020-10-01 15:05:00,2020-10-01 16:25:00
2020-10-01 19:15:00	2020-10-01 19:30:00.000000	30	2020-10-01 16:10:00,2020-10-01 16:25:00
2020-10-01 19:30:00	2020-10-01 19:45:00.000000	30	2020-10-01 16:10:00,2020-10-01 16:25:00
2020-10-01 19:45:00	2020-10-01 20:00:00.000000	30	2020-10-01 16:10:00,2020-10-01 16:25:00
2020-10-01 20:00:00	2020-10-01 20:15:00.000000	30	2020-10-01 16:10:00,2020-10-01 16:25:00
2020-10-01 20:15:00	2020-10-01 20:30:00.000000	30	2020-10-01 16:10:00,2020-10-01 16:25:00
2020-10-01 20:30:00	2020-10-01 20:45:00.000000	30	2020-10-01 16:10:00,2020-10-01 16:25:00
2020-10-01 20:45:00	2020-10-01 21:00:00.000000	30	2020-10-01 16:10:00,2020-10-01 16:25:00
2020-10-01 21:00:00	2020-10-01 21:15:00.000000	30	2020-10-01 16:10:00,2020-10-01 16:25:00
2020-10-01 21:15:00	2020-10-01 21:30:00.000000	30	2020-10-01 16:25:00,2020-10-01 16:10:00
2020-10-01 21:30:00	2020-10-01 21:45:00.000000	30	2020-10-01 16:25:00,2020-10-01 16:10:00
2020-10-01 21:45:00	2020-10-01 22:00:00.000000	30	2020-10-01 16:25:00,2020-10-01 16:10:00
2020-10-01 22:00:00	2020-10-01 22:15:00.000000	30	2020-10-01 16:25:00,2020-10-01 16:10:00
2020-10-01 22:15:00	2020-10-01 22:30:00.000000	25	2020-10-01 16:10:00,2020-10-01 16:25:00
2020-10-01 22:30:00	2020-10-01 22:45:00.000000	15	2020-10-01 16:10:00
2020-10-01 22:45:00	2020-10-01 23:00:00.000000	15	2020-10-01 16:10:00
2020-10-01 23:00:00	2020-10-01 23:15:00.000000	15	2020-10-01 16:10:00
2020-10-01 23:15:00	2020-10-01 23:30:00.000000	15	2020-10-01 16:10:00
2020-10-01 23:30:00	2020-10-01 23:45:00.000000	15	2020-10-01 16:10:00
2020-10-01 23:45:00	2020-10-02 00:00:00.000000	15	2020-10-01 16:10:00
2020-10-02 00:00:00	2020-10-02 00:15:00.000000	10	2020-10-01 16:10:00