MySQL 8.0

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CREATE TABLE srctable
SELECT 1 callnumber, '2019-01-01 10:01:23' starttime, '2019-01-01 10:05:44' endtime
UNION ALL
SELECT 2, '2019-01-01 10:02:33', '2019-01-01 10:05:22'
UNION ALL
SELECt 3, '2019-01-01 10:03:50', '2019-01-01 10:06:11'

Records: 3  Duplicates: 0  Warnings: 0


select * from srctable


callnumber
starttime
endtime


1
2019-01-01 10:01:23
2019-01-01 10:05:44

2
2019-01-01 10:02:33
2019-01-01 10:05:22

3
2019-01-01 10:03:50
2019-01-01 10:06:11


WITH RECURSIVE 
cte1 AS ( SELECT CAST(DATE_FORMAT(MIN(StartTime), '%Y-%m-%d %H:%i:00') AS DATETIME) mintime,
                 CAST(DATE_FORMAT(MAX(EndTime), '%Y-%m-%d %H:%i:00') AS DATETIME) maxtime
                 FROM srctable ),
cte2 AS ( SELECT mintime timechunk, 
                 mintime + INTERVAL 1 MINUTE nextchunk
          FROM cte1
        UNION ALL
          SELECT cte2.nextchunk, 
                 cte2.nextchunk + INTERVAL 1 MINUTE
          FROM cte1, cte2
          WHERE cte2.timechunk < cte1.maxtime )
SELECT cte2.timechunk time_chunk, 
       COUNT(*) calls_in_progress
FROM cte2
LEFT JOIN srctable ON cte2.timechunk < srctable.endtime
                  AND cte2.nextchunk >= srctable.starttime
GROUP BY cte2.timechunk 
WITH ROLLUP 
ORDER BY cte2.timechunk IS NULL, cte2.timechunk 


time_chunk
calls_in_progress


2019-01-01 10:01:00
1

2019-01-01 10:02:00
2

2019-01-01 10:03:00
3

2019-01-01 10:04:00
3

2019-01-01 10:05:00
3

2019-01-01 10:06:00
1

null
13

callnumber	starttime	endtime
1	2019-01-01 10:01:23	2019-01-01 10:05:44
2	2019-01-01 10:02:33	2019-01-01 10:05:22
3	2019-01-01 10:03:50	2019-01-01 10:06:11

time_chunk	calls_in_progress
2019-01-01 10:01:00	1
2019-01-01 10:02:00	2
2019-01-01 10:03:00	3
2019-01-01 10:04:00	3
2019-01-01 10:05:00	3
2019-01-01 10:06:00	1
null	13