SQL COUNT number of patients each month - sql

Question

I have a table with:
PATIENT_ID
START_DATE
END_DATE
Ward
1
19/01/2022
19/02/2022
A
2
20/01/2022
19/03/2022
A
And I want to create a summarized table to show for each month, how many patients were active in that ward as well as the total number of patient days for that month. Is this possible in SQL?
I'm thinking I might need an external DIM_DATE table that has all of the months up until now and starts from the first START_DATE out of all the PATIENT_ID's but doesn't sound very efficient?
Note that I have shown only 1 ward but there are also different wards.
Expected result:
Month
Ward
COUNT_PATIENTS
TOTAL_NUMBER_DAYS
31/01/2022
A
2
33
28/02/2022
A
2
47
31/03/2022
A
1
19

Answer 1

with data(PATIENT_ID, START_DATE, END_DATE, Ward) as (
select column1, to_date(column2, 'dd/mm/yyyy'), to_date(column3, 'dd/mm/yyyy'), column4
from values
(1, '19/01/2022','19/02/2022','A'),
(2, '20/01/2022','19/03/2022','A')
), ranges as (
select
date_trunc('month', min(start_date)) as min_start,
dateadd('day', -1, dateadd('month', 1, date_trunc('month', max(end_date)))) as max_end
from data
), gen as (
select
row_number() over(order by null)-1 as rn
from table(generator(ROWCOUNT => 1000))
), all_months as (
select
dateadd('month', g.rn, date_trunc(month, r.min_start)) as month_start,
dateadd('day', -1, dateadd('month', 1, month_start)) as month_end
from ranges as r
cross join gen as g
qualify month_start <= r.max_end
)
select
a.month_end as month,
d.ward,
a.month_end))+1 as days
count(distinct patient_id) as count_patients,
sum(datediff(days, greatest(d.start_date, a.month_start), least(d.END_DATE, a.month_end))+1) as total_numbers_days
from all_months as a
left join data as d
on a.month_start between date_trunc('month', d.START_DATE) and date_trunc('month', d.END_DATE)
group by 1,2
order by 1,2
gives:
MONTH
WARD
COUNT_PATIENTS
TOTAL_NUMBERS_DAYS
2022-01-31
A
2
25
2022-02-28
A
2
47
2022-03-31
A
1
19
I think your 33 is wrong, as the partials are:
MONTH
WARD
PATIENT_ID
_START
_END
DAYS
2022-01-31
A
1
2022-01-19
2022-01-31
13
2022-01-31
A
2
2022-01-20
2022-01-31
12
2022-02-28
A
1
2022-02-01
2022-02-19
19
2022-02-28
A
2
2022-02-01
2022-02-28
28
2022-03-31
A
2
2022-03-01
2022-03-19
19