How to achieve partition by kind of functionality using variables in MySQL 5.7

I’m using an old version of MySQL. I cannot upgrade it due to some strong reasons. I’ve a table PatientAppointment with only 2 columns. Here is the table:

+---------------------+-----------------+
| dateOfCheckup       | duration        |
+---------------------+-----------------+
| 2020-05-28          |   30 min        |
| 2020-05-29          |   30 min        |
| 2020-05-30          |   1 hour        |
| 2020-06-03          |   1 hour        |
| 2020-06-05          |   1 hour 30 min |
| 2020-07-21          |   1 hour 30 min |
| 2020-07-22          |   1 hour        |
| 2020-07-28          |   30 min        |
+---------------------+-----------------+

I want 5 things:

1. Create a new column in runtime monthOfCheckup from dateOfCheckup
2. Convert duration into integer field as minutes
3. Accumulating total for each month (not date)

I was able to achieve these 3 points with this query:

select
dateOfCheckup,
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(dateOfCheckup, '%Y-%m')
     THEN @cum_sum := @cum_sum + @cur_dur
     ELSE @cum_sum := @cur_dur
     END total,
@year_month := date_format(dateOfCheckup, '%Y-%m') monthOfCheckup

from patient, (SELECT @year_month:='', @cum_sum:=0, @cur_dur:=0) variables
ORDER BY dateOfCheckup

Here is the fiddle for the same.

Now the remaining 2 points are:

4. BillElig1 This can be boolean or text, doesn’t matters. I just have to put ‘Y’ if the total for that particular month is >=20
5. BillElig2 This can be boolean or text, doesn’t matters. I just have to put ‘Y’ if the total for that particular month is >=40

For these 2 points you can see this fiddle.

Please help me.

Answer

And again use UDV.

SELECT dateOfCheckup,
       duration,
       minutes,
       CASE WHEN @prev_month != monthOfCheckup
            THEN total >= 20
            WHEN @prev_total < 20 
            THEN 1
            ELSE 0 
            END 99457Elig,
       CASE WHEN @prev_month != monthOfCheckup
            THEN total >= 40
            WHEN @prev_total < 40
            THEN 1
            ELSE 0 
            END 99458Elig,
       @prev_month := monthOfCheckup monthOfCheckup,
       @prev_total := total total
FROM (select dateOfCheckup,
             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(dateOfCheckup, '%Y-%m')
                  THEN @cum_sum := @cum_sum + @cur_dur
                  ELSE @cum_sum := @cur_dur
                  END total,
             @year_month := date_format(dateOfCheckup, '%Y-%m') monthOfCheckup

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

fiddle

I use 1 and 0 instead of Y and NULL.

Of course, the columns order in output list and ordering are critical again.