Measure population on several dates - sql

Question

I want to measure the population of our manucipality (which contains out of several places). I've got two tables in: my first dataset is a calender table with a row for each first day of every month.
My second table contains alle the people that live and have lived in the manucipality.
What I want is the population of each place on every first day of the month from my calender table. I've put some raw data below (just a few records of the persons table because it contains 100.000 records)
Calender table:
+----------+
| Date |
+----------+
| 1-1-2018 |
+----------+
| 1-2-2018 |
+----------+
| 1-3-2018 |
+----------+
| 1-4-2018 |
+----------+
Persons table
+-----+-----------+-----------+---------------+-------+
| BSN | Startdate | Enddate | Date of death | Place |
+-----+-----------+-----------+---------------+-------+
| 1 | 12-1-2000 | null | null | A |
+-----+-----------+-----------+---------------+-------+
| 2 | 10-5-2011 | null | 22-1-2018 | B |
+-----+-----------+-----------+---------------+-------+
| 3 | 16-12-2011| 10-2-2018 | null | B |
+-----+-----------+-----------+---------------+-------+
| 4 | 9-11-2012 | null | null | B |
+-----+-----------+-----------+---------------+-------+
| 5 | 8-9-2013 | null | 27-3-2018 | A |
+-----+-----------+-----------+---------------+-------+
| 6 | 7-10-2017 | 28-3-2018 | null | B |
+-----+-----------+-----------+---------------+-------+
My expected result:
+----------+-------+------------+
| Date | Place | Population |
+----------+-------+------------+
| 1-1-2018 | A | 2 |
+----------+-------+------------+
| 1-1-2018 | B | 4 |
+----------+-------+------------+
| 1-2-2018 | A | 2 |
+----------+-------+------------+
| 1-2-2018 | B | 3 |
+----------+-------+------------+
| 1-3-2018 | A | 2 |
+----------+-------+------------+
| 1-3-2018 | B | 2 |
+----------+-------+------------+
| 1-4-2018 | A | 1 |
+----------+-------+------------+
| 1-4-2018 | B | 1 |
+----------+-------+------------+
What I've done so far but doesnt seems to work:
SELECT a.Place
,c.Date
,(SELECT COUNT(DISTINCT(b.BSN))
FROM Person as b
WHERE b.Startdate < c.Date
AND (b.Enddate > c.Date OR b.Enddate is null)
AND (b.Date of death > c.Date OR b.Date of death is null)
AND a.Place = b.Place) as Population
FROM Person as a
JOIN Calender as c
ON a.Startdate <= c.Date
AND a.Enddate >= c.Date
GROUP BY Place, Date
I hope someone can help finding out the problem. Thanks in advance

Answer 1

First cross join Calender and the places to get the date/place pairs. Then left join the persons on the place and the date. Finally group by date and place to get the count of people for that day and place.
SELECT [ca].[Date],
[pl].[Place],
count([pe].[Place]) [Population]
FROM [Calender] [ca]
CROSS JOIN (SELECT DISTINCT
[pe].[Place]
FROM [Persons] [pe]) [pl]
LEFT JOIN [Persons] [pe]
ON [pe].[Place] = [pl].[Place]
AND [pe].[Startdate] <= [ca].[Date]
AND (colaesce([pe].[Enddate],
[pe].[Date of death]) IS NULL
OR coalesce([pe].[Enddate],
[pe].[Date of death]) > [ca].[Date])
GROUP BY [ca].[Date],
[pl].[Place]
ORDER BY [ca].[Date],
[pl].[Place];
Some notes and assumptions:
If you have a table listing the places use that instead of the subquery aliases [pl]. I just had no other option with the given tables.
I believe the Date of death also implies an Enddate for the same day. You might want to consider a trigger, that sets the Enddate automatically to the Date of death if it isn't null. That would make things easier and probably more consistent.