PostgreSQL - how to loop over a parameter to make union of different queries of the same table - sql

Question

I need to count the number of events still occurring in a given year, provided these events started before the given year and ended after it. So, the query I'm using now is just a pile of unparametrized queries and I'd like to write it out with a loop on a parameter:
select
count("starting_date" ) as "number_of_events" ,'2020' "year"
from public.table_of_events
where "starting_date" < '2020-01-01' and ("closing_date" > '2020-12-31')
union all
select
count("starting_date" ) as "number_of_events" ,'2019' "year"
from public.table_of_events
where "starting_date" < '2019-01-01' and ("closing_date" > '2019-12-31')
union all
select
count("starting_date" ) as "number_of_events" ,'2018' "year"
from public.table_of_events
where "starting_date" < '2018-01-01' and ("closing_date" > '2018-12-31')
...
...
...
and so on for N years
So, I have ONE table that must be filtered according to one parameter that is both part of the select statement and the where clause
The general aspect of the "atomic" query is then
select
count("starting_date" ) as "number_of_events" , **PARAMETER** "year"
from public.table_of_events
where "starting_date" < **PARAMETER** and ("closing_date" > **PARAMETER** )
union all
Can anyone help me put this in a more formal loop?
Thanks a lot, I hope I was clear enough.

Answer 1

You seem to want events that span entire years. Perhaps a simple way is to generate the years, then use join and aggregate:
select gs.yyyy, count(e.starting_date)
from public.table_of_events e left join
generate_series('2015-01-01'::date,
'2021-01-01'::date,
interval '1 year'
) as gs(yyyy)
on e.starting_date < gs.yyyy and
e.closing_date >= gs.yyyy + interval '1 year'
group by gs.yyyy;