Counting distinct members based on number of months in duration of time - sql

Question

I have a membership table with the following columns:
Member_number | StartDate | EndDate
XYZ | 01-Jan-2002 | 01-March-2002
ABC | 01-Feb-2002 | 01-March-2002
Basically, I want to show how many members were present in specific month. My problem is I don't know how to break this time span into months. How can I see this result?
Month | NumberOfMembers
Jan | 1
Feb | 2
March | 2

Answer 1

Given a members table that looks something like this:
create table dbo.members
(
member_number int not null primary key ,
start_date datetime not null ,
end_date datetime not null ,
)
And a table-valued function that generates sequences of consecutive integers, like this:
create function dbo.IntegerRange ( #from int , #thru int )
returns #sequence table
(
value int not null primary key clustered
)
as
begin
declare #increment int = case when #from > #thru then -1 else 1 end ;
with sequence(value) as
(
select value = #from
union all
select value + #increment
from sequence
where value < #thru
)
insert #sequence
select value
from sequence
order by value
option ( MAXRECURSION 0 )
return
end
A query like this should give you what you want:
select [year] = period.yyyy ,
[month] = case period.mm ,
when 1 then 'Jan'
when 2 then 'Feb'
when 3 then 'Mar'
when 4 then 'Apr'
when 5 then 'May'
when 6 then 'Jun'
when 7 then 'Jul'
when 8 then 'Aug'
when 9 then 'Sep'
when 10 then 'Oct'
when 11 then 'Nov'
when 12 then 'Dev'
else '***'
end ,
member_cnt = sum( case when m.member_number is not null then 1 else 0 end )
from ( select yyyy = yyyy.value ,
mm = mm.value ,
dtFrom = dateadd( month , mm.value - 1 , dateadd( year , yyyy.value - 1900 , convert(date,'') ) ) ,
dtThru = dateadd( day , - 1 , dateadd( month , mm.value , dateadd( year , yyyy.value - 1900 , convert(date,'') ) ) )
from dbo.IntegerRange(2000,2013) yyyy
full join dbo.IntegerRange(1,12) mm on 1=1
) period
left join dbo.members m on period.dtFrom <= m.end_date
and period.dtThru >= m.start_date
group by period.yyyy ,
period.mm
order by period.yyyy ,
period.mm
The first table expression in the from clause creates a virtual table of the periods (months, in this case, but the technique doesn't limit itself to months or even weeks) covering the reporting period:
from ( select yyyy = yyyy.value ,
mm = mm.value ,
dtFrom = dateadd( month , mm.value - 1 , dateadd( year , yyyy.value - 1900 , convert(date,'') ) ) ,
dtThru = dateadd( day , - 1 , dateadd( month , mm.value , dateadd( year , yyyy.value - 1900 , convert(date,'') ) ) )
from dbo.IntegerRange(2000,2013) yyyy
full join dbo.IntegerRange(1,12) mm on 1=1
) period
That is then joined, via a left outer join, ensuring that all periods are reported, not just those periods with active members, to the members table to collect, for each reporting period in the virtual period table above, the set of members who were active during the period:
left join dbo.members m on period.dtFrom <= m.end_date
and period.dtThru >= m.start_date
We then group by the year and month of each period and then order the results by year/month number:
group by period.yyyy ,
period.mm
order by period.yyyy ,
period.mm
In creating the results set to be returned, we return the year of the period, the month number (converted to a friendly name), and the count of active members. Note that we have to use the sum() aggregate function here rather than count() as empty periods will have a single row returned (with null in all columns). Count(), unlike all other aggregate functions, includes null values in the aggregation. Sum() is applied to a case expression acting as a discriminant function returning 1 or 0 identifying whether the row indicates useful or missing data:
select [year] = period.yyyy ,
[month] = case period.mm ,
when 1 then 'Jan'
when 2 then 'Feb'
when 3 then 'Mar'
when 4 then 'Apr'
when 5 then 'May'
when 6 then 'Jun'
when 7 then 'Jul'
when 8 then 'Aug'
when 9 then 'Sep'
when 10 then 'Oct'
when 11 then 'Nov'
when 12 then 'Dev'
else '***'
end ,
member_cnt = sum( case when m.member_number is not null then 1 else 0 end )
Easy!

Answer 2

DECLARE #minMonth DATE
SELECT #minMonth = MIN(StartDate) FROM Table1
DECLARE #maxMonth DATE
SELECT #maxMonth = MAX(EndDate) FROM Table1
;WITH CTE_Months AS
(
SELECT #minMonth AS Mnth
UNION ALL
SELECT DATEADD(MM,1,Mnth) FROM CTE_Months
WHERE Mnth<#MaxMonth
)
SELECT Mnth AS Month, COUNT(*) as Members
FROM CTE_Months m
LEFT JOIN Table1 t on m.Mnth BETWEEN t.StartDate AND t.EndDate
GROUP BY Mnth
SQLFiddle Demo
CTE will find all months from min StartDate to max EndDate, if you need different min and max, just change how you get #MinMonth and #MaxMonth
If you don't want to show zeros for months that possibly don't have any members, replace LEFT JOIN with INNER at the end.

Answer 3

select t.Month, count(*) Members
from (
select case
when startdate <= to_date('01-Jan-2002', 'DD-MON-YYYY') AND enddate >= to_date('31-Jan-2002', 'DD-MON-YYYY') then ' Jan'
when startdate <= to_date('01-Feb-2002', 'DD-MON-YYYY') AND enddate >= to_date('28-Feb-2002', 'DD-MON-YYYY') then ' Feb'
when startdate <= to_date('01-Mar-2002', 'DD-MON-YYYY') AND enddate >= to_date('31-Mar-2002', 'DD-MON-YYYY') then ' Mar'
when startdate <= to_date('01-Apr-2002', 'DD-MON-YYYY') AND enddate >= to_date('30-Apr-2002', 'DD-MON-YYYY') then ' Apr'
when startdate <= to_date('01-May-2002', 'DD-MON-YYYY') AND enddate >= to_date('31-May-2002', 'DD-MON-YYYY') then ' May'
when startdate <= to_date('01-Jun-2002', 'DD-MON-YYYY') AND enddate >= to_date('30-Jun-2002', 'DD-MON-YYYY') then ' Jun'
when startdate <= to_date('01-Jul-2002', 'DD-MON-YYYY') AND enddate >= to_date('31-Jul-2002', 'DD-MON-YYYY') then ' Jul'
when startdate <= to_date('01-Aug-2002', 'DD-MON-YYYY') AND enddate >= to_date('31-Aug-2002', 'DD-MON-YYYY') then ' Aug'
when startdate <= to_date('01-Sep-2002', 'DD-MON-YYYY') AND enddate >= to_date('30-Sep-2002', 'DD-MON-YYYY') then ' Sep'
when startdate <= to_date('01-Oct-2002', 'DD-MON-YYYY') AND enddate >= to_date('31-Oct-2002', 'DD-MON-YYYY') then ' Oct'
when startdate <= to_date('01-Nov-2002', 'DD-MON-YYYY') AND enddate >= to_date('30-Nov-2002', 'DD-MON-YYYY') then ' Nov'
when startdate <= to_date('01-Dec-2002', 'DD-MON-YYYY') AND enddate >= to_date('31-Dec-2002', 'DD-MON-YYYY') then ' Dec'
end as Month
from member) t
group by t.Month
Where member is the table name.