MySQL 8.0

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create table patient (dateOfCheckup date, duration varchar(20))
insert into patient values('2020-05-28', '30 min');
insert into patient values('2020-05-29', '30 min');
insert into patient values('2020-05-30', '1 hour');
insert into patient values('2020-06-03', '1 hour 30 min');
insert into patient values('2020-06-05', '30 min');
insert into patient values('2020-07-21', '1 hour');
insert into patient values('2020-07-22', '1 hour 30 min');
insert into patient values('2020-07-28', '1 hour 30 min');
select dateOfCheckup, duration, ((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 from patient;


dateOfCheckup
duration
minutes


2020-05-28
30 min
30

2020-05-29
30 min
30

2020-05-30
1 hour
60

2020-06-03
1 hour 30 min
90

2020-06-05
30 min
30

2020-07-21
1 hour
60

2020-07-22
1 hour 30 min
90

2020-07-28
1 hour 30 min
90

dateOfCheckup	duration	minutes
2020-05-28	30 min	30
2020-05-29	30 min	30
2020-05-30	1 hour	60
2020-06-03	1 hour 30 min	90
2020-06-05	30 min	30
2020-07-21	1 hour	60
2020-07-22	1 hour 30 min	90
2020-07-28	1 hour 30 min	90