Grouping of actvie events given Start and End date (SQL Server) - sql

Question

I have the challenge that I have a table with start and end date of an event:
Event Start End
A 01Jan2018 01Mar2018
B 01Feb2018 01Apr2018
I would like to have a table as output with a group by of active events in a month:
Year Month count_active_events
2018 1 1
2018 2 2
2018 3 2
2018 4 1
Can anyone think of a SQL statement that makes this request feasible?
THX
Lazloo

Answer 1

One method generates the dates and then does the calculation using a correlated subquery or apply:
with dates as (
select cast('2018-01-01' as date) as dte
union all
select dateadd(month, 1, dte)
from dates
where dte < '2018-04-01'
)
select d.dte,
(select count(*)
from t
where t.start <= d.dte and t.end >= d.dte
) as num_active
from dates d;

Answer 2

Try this:
declare #tbl table([Event] char(1), [Start] date , [End] date);
insert into #tbl Values
('A' , '01Jan2018', '01Mar2018'),
('B' , '01Feb2018', '01Apr2018');
declare #start date,
#end date;
select #start = min([Start]), #end = max([End]) from #tbl;
with cte as(
select #start dt from #tbl
union all
select dateadd(month, 1, dt) from cte
where dateadd(month, 1, dt) <= #end
)
select datepart(month, c.dt), datepart(year, c.dt), count(distinct [Event]) from cte c
left join #tbl t on c.dt between t.Start and t.[End]
group by datepart(month, c.dt), datepart(year, c.dt)

Answer 3

You need apply with recursive cte to generate start event dates :
with tt1 as (
select event, cast(start as date) start, cast([end] as date) [end]
from table
union all
select event, dateadd(month, 1, start), [end]
from tt1
where start < [end]
)
select year(tt.dt), month(tt.dt), tt1.cnt
from table t cross apply
( values (start), ([end])
) tt (dt) outer apply
( select count(*) cnt
from tt1
where year(tt1.start) = year(tt.dt) and
month(tt1.start) = month(tt.dt)
) tt1;