Member Data between date ranges - sql

Question

I have a table in SQL Server 2014 named [Membership] containing personal member data and two date fields named [member from date] and [member to date].
I need to summarise the monthly membership. A member is counted in a given month only if they are a member for that whole month.
So for example, a person with [member from date] of '2014-02-01' and [member to date] of '2015-03-01' would be counted in the month of December 2014, but would not be if the [member to date] was, say, '2014-12-25'.
I need to summarise by every month going back to January 2010 and I have thousands of members in this table. The results need to look similar to this:
Month Count
Jan 2010 3230
Feb 2010 3235
Mar 2010 3232
..
Dec 2016 6279
I can't see how to work this because of the "only if they are a member for that whole month" rule.
Any help will be most appreciated!

Answer 1

Using spt_values and a cte to generate the calendar, here is an example that counts members.
declare #members table (member int, start_date date, end_date date)
insert #members select 1, '2015-12-15', '2017-01-15'
insert #members select 2, '2016-01-15', '2016-12-15'
insert #members select 3, '2016-03-01', '2016-10-31'
declare #cal_from datetime = '2016-01-01';
declare #cal_to datetime = '2016-12-31';
with calendar_cte as (
select top (datediff(month, #cal_from, #cal_to) + 1)
[Month] = month(dateadd(month, number, #cal_from))
, [Year] = year(dateadd(month, number, #cal_from))
, [Start] = dateadd(month, number, #cal_from)
, [End] = dateadd(day, -1, dateadd(month, number + 1, #cal_from))
from [master].dbo.spt_values
where [type] = N'P'
order by number
)
select [Month]
, [Year]
, [Count] = (select count(*)
from #members
where start_date <= [Start]
and end_date >= [End])
from calendar_cte

Answer 2

With the help of a Months table, this can be handled pretty easily:
/* creating a months table */
create table dbo.Months([Month] date primary key, MonthEnd date);
declare #StartDate date = '20100101'
,#NumberOfYears int = 30;
insert dbo.Months([Month],MonthEnd)
select top (12*#NumberOfYears)
[Month] = dateadd(month, row_number() over (order by number) -1, #StartDate)
, MonthEnd = dateadd(day,-1,
dateadd(month, row_number() over (order by number), #StartDate)
)
from master.dbo.spt_values;
/* the query */
select [Month], [Count]=count(*)
from dbo.Months mo
inner join dbo.[Membership] me on
/* Member since the start of the month */
me.MemberFromDate >= mo.[Month]
/* Member for the entire month being counted */
and me.MemberToDate > mo.[MonthEnd]
group by [Month]
order by [Month]
If you really don't want to have a Months table, you can use a cte like this:
declare #StartDate date = '20100101'
,#NumberOfYears int = 30;
;with Months as (
select top (12*#NumberOfYears)
[Month] = dateadd(month, row_number() over (order by number) -1, #StartDate)
, MonthEnd = dateadd(day,-1,
dateadd(month, row_number() over (order by number), #StartDate)
)
from master.dbo.spt_values
)
/* the query */
select [Month], [Count]=count(*)
from Months mo
inner join dbo.[Membership] me on
/* Member since the start of the month */
me.MemberFromDate >= mo.[Month]
/* Member for the entire month being counted */
and me.MemberToDate > mo.[MonthEnd]
group by [Month]
order by [Month]

Answer 3

create table members(name varchar(50),fromdate datetime, todate datetime)
go
create table months(firstday datetime)
go
insert members values('Joe','2014-02-01','2015-03-25'),('Jon','2014-03-12','2015-01-12')
declare #date datetime = '2000-01-01'
while (#date < '2016-01-01')
begin
insert into months values( #date )
select #date = dateadd(month,1,#date)
end
with MyCTE(date) as
( select left(convert(varchar, firstday, 120),7)
from members m
join months d on d.firstday > m.fromdate and d.firstday < datefromparts(year(m.todate),month(m.todate),1)
)
select date as 'month', count(*) as 'count'
from MyCTE
group by date