MySQL 5.7

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create table PatientAppointment (CDRId varchar(20), AppointmentDateTime date, duration varchar(20))
insert into PatientAppointment values('Person1','2020-05-28', '30 min');
insert into PatientAppointment values('Person1','2020-05-29', '30 min');
insert into PatientAppointment values('Person1','2020-05-30', '2 hour');
insert into PatientAppointment values('Person1','2020-06-03', '1 hour 30 min');
insert into PatientAppointment values('Person1','2020-06-05', '30 min');
insert into PatientAppointment values('Person1','2020-07-21', '1 hour 20 min');
insert into PatientAppointment values('Person1','2020-07-22', '30 min');
insert into PatientAppointment values('Person1','2020-07-28', '1 hour 15 min');
insert into PatientAppointment values('Person2','2020-05-23', '30 min');
insert into PatientAppointment values('Person2','2020-06-20', '30 min');
insert into PatientAppointment values('Person2','2020-06-23', '2 hour');
insert into PatientAppointment values('Person2','2020-07-11', '1 hour 30 min');
insert into PatientAppointment values('Person2','2020-07-15', '30 min');
insert into PatientAppointment values('Person2','2020-08-23', '2 hour 30 min');
insert into PatientAppointment values('Person2','2020-08-24', '30 min');
insert into PatientAppointment values('Person2','2020-08-29', '1 hour 30 min');
insert into PatientAppointment values('Person3','2020-03-11', '30 min');
insert into PatientAppointment values('Person3','2020-03-12', '30 min');
insert into PatientAppointment values('Person3','2020-03-13', '3 hour');
insert into PatientAppointment values('Person3','2020-04-20', '1 hour 30 min');
insert into PatientAppointment values('Person3','2020-04-23', '30 min');
insert into PatientAppointment values('Person3','2020-05-21', '1 hour 10 min');
insert into PatientAppointment values('Person3','2020-05-25', '30 min');
insert into PatientAppointment values('Person3','2020-05-29', '1 hour 30 min');
SELECT CDRId,
       AppointmentDateTime,
       duration,
       minutes,
       @prev_month := BillingMonth BillingMonth,
       @prev_total := total total
FROM (select CDRId, AppointmentDateTime,
             duration,
             @cur_dur := ((case when duration like '% hour%' then substring_index(duration, ' hour', 1) * 60 else 0 end) +
                         (case when duration like '%min%' then substring_index(substring_index(duration, ' min', 1), ' ', -1) + 0 else 0 end)) as minutes,

             CASE WHEN @year_month = date_format(AppointmentDateTime, '%Y-%m')
                  THEN @cum_sum := @cum_sum + @cur_dur
                  ELSE @cum_sum := @cur_dur
                  END total,
             @year_month := date_format(AppointmentDateTime, '%Y-%m') BillingMonth

      from PatientAppointment, (SELECT @year_month:='', @cum_sum:=0, @cur_dur:=0) variables
      ORDER BY AppointmentDateTime) subquery, 
(SELECT @prev_month:=0, @prev_total:=0) variable
ORDER BY AppointmentDateTime


CDRId
AppointmentDateTime
duration
minutes
BillingMonth
total


Person3
2020-03-11
30 min
30
2020-03
30

Person3
2020-03-12
30 min
30
2020-03
60

Person3
2020-03-13
3 hour
180
2020-03
240

Person3
2020-04-20
1 hour 30 min
90
2020-04
90

Person3
2020-04-23
30 min
30
2020-04
120

Person3
2020-05-21
1 hour 10 min
70
2020-05
70

Person2
2020-05-23
30 min
30
2020-05
100

Person3
2020-05-25
30 min
30
2020-05
130

Person1
2020-05-28
30 min
30
2020-05
160

Person1
2020-05-29
30 min
30
2020-05
190

Person3
2020-05-29
1 hour 30 min
90
2020-05
280

Person1
2020-05-30
2 hour
120
2020-05
400

Person1
2020-06-03
1 hour 30 min
90
2020-06
90

Person1
2020-06-05
30 min
30
2020-06
120

Person2
2020-06-20
30 min
30
2020-06
150

Person2
2020-06-23
2 hour
120
2020-06
270

Person2
2020-07-11
1 hour 30 min
90
2020-07
90

Person2
2020-07-15
30 min
30
2020-07
120

Person1
2020-07-21
1 hour 20 min
80
2020-07
200

Person1
2020-07-22
30 min
30
2020-07
230

Person1
2020-07-28
1 hour 15 min
75
2020-07
305

Person2
2020-08-23
2 hour 30 min
150
2020-08
150

Person2
2020-08-24
30 min
30
2020-08
180

Person2
2020-08-29
1 hour 30 min
90
2020-08
270

CDRId	AppointmentDateTime	duration	minutes	BillingMonth	total
Person3	2020-03-11	30 min	30	2020-03	30
Person3	2020-03-12	30 min	30	2020-03	60
Person3	2020-03-13	3 hour	180	2020-03	240
Person3	2020-04-20	1 hour 30 min	90	2020-04	90
Person3	2020-04-23	30 min	30	2020-04	120
Person3	2020-05-21	1 hour 10 min	70	2020-05	70
Person2	2020-05-23	30 min	30	2020-05	100
Person3	2020-05-25	30 min	30	2020-05	130
Person1	2020-05-28	30 min	30	2020-05	160
Person1	2020-05-29	30 min	30	2020-05	190
Person3	2020-05-29	1 hour 30 min	90	2020-05	280
Person1	2020-05-30	2 hour	120	2020-05	400
Person1	2020-06-03	1 hour 30 min	90	2020-06	90
Person1	2020-06-05	30 min	30	2020-06	120
Person2	2020-06-20	30 min	30	2020-06	150
Person2	2020-06-23	2 hour	120	2020-06	270
Person2	2020-07-11	1 hour 30 min	90	2020-07	90
Person2	2020-07-15	30 min	30	2020-07	120
Person1	2020-07-21	1 hour 20 min	80	2020-07	200
Person1	2020-07-22	30 min	30	2020-07	230
Person1	2020-07-28	1 hour 15 min	75	2020-07	305
Person2	2020-08-23	2 hour 30 min	150	2020-08	150
Person2	2020-08-24	30 min	30	2020-08	180
Person2	2020-08-29	1 hour 30 min	90	2020-08	270