MSSQL Date Range Report - Count of days per month - sql-server

Question

I have a table of instances that have a Start Date and an End Date column. Here is a simple example:
ID StartDate EndDate
1 1/8/2015 1/10/2015
2 1/8/2015 1/15/2015
3 2/6/2015 3/2/2015
4 1/6/2015 2/20/2015
5 3/18/2015 4/2/2015
I'm trying to write a query to find out how many unique days occur for a given month, but some of the instances overlap and span multiple months which is making it difficult. The results I want would look something like this:
Month # of days
January 26 (earliest is ID 4 starting 1/6)
February 28 (entire month because of ID 3 and 4)
March 16 (2 days from ID 3, 14 days from ID 5)
April 2 (first 2 days of the month from ID 5)
May 0
Any help would be greatly appreciated. Thanks!!

Answer 1

J,
Please check my SQL script below.
Before you run the script you will realize that I've used a SQL Dates table actually a SQL function which returns a temporary dates table.
You can find the source codes at given tutorial
I also used multiple CTE queries
;with dates as (
select
cast(date as date) date
from [dbo].[DateTable]('1/1/2015','12/31/2015')
), cte as (
select
distinct date
from instances, dates
where dates.date between instances.startdate and instances.enddate
)
select
year(date) year, month(date) month, count(*) dayscount
from cte
group by year(date), month(date)
By the way the March returns 16 days, 2 from one and 14 from other.
I hope the Select statement is useful,

Answer 2

The problem is too complicated;
In my opinion, you need to write a function that counts the number of "unique" days of ranges mentioned in the records.
I didn't write the function, but the design of this new function, "num", is like this:
1- It should get the month and year (named aMonth and aYear).
2- It Finds all records that have at least a day in aYear/aMonth:
(month(startDate)=aMonth and year(startDate)=aYear)
or
(month(endDate)=aMonth and year(endDate)=aYear)
or
(
((month(startDate)<aMonth and year(startDate)=aYear) or (year(startDate)<aYear))
and
((month(endDate)>aMonth and year(endDate)=aYear) or (year(endDate)>aYear))
)
3- Over these records, it should open a cursor, and process the records one by one.
4- While processing each records of the cursor, you can count the days of the month and store them in an array (or 28-31 character string of 0/1, for example).
5- count the number of 1's of this array (or string) and return it.
Having written this function ("num"), The high level of the answer will be like this:
Select 'January', dbo.num(1, 2015) as days
union all
Select 'February', dbo.num(2, 2015) as days
union all
Select 'March', dbo.num(3, 2015) as days
union all
Select 'April', dbo.num(4, 2015) as days
union all
Select 'May', dbo.num(5, 2015) as days
union all
Select 'June', dbo.num(6, 2015) as days
union all
Select 'July', dbo.num(7, 2015) as days
union all
Select 'August', dbo.num(8, 2015) as days
union all
Select 'September', dbo.num(9, 2015) as days
union all
Select 'October', dbo.num(10, 2015) as days
union all
Select 'November', dbo.num(11, 2015) as days
union all
Select 'December', dbo.num(12, 2015) as days

Answer 3

If you count the days for the same year only, you can try this. I only build the code for two months but it's easy to extend it.
SELECT
(SELECT SUM(CASE WHEN sdate>='2015-2-1' OR edate<'2015-1-1' THEN 0
WHEN edate>='2015-2-1' THEN datediff(day, sdate, '2015-2-1')
ELSE datediff(day,sdate,edate) END)
FROM a1)
AS Jan_Days,
(SELECT SUM(CASE WHEN sdate>='2015-3-1' OR edate<'2015-2-1' THEN 0
WHEN edate>='2015-3-1' THEN datediff(day, sdate, '2015-3-1')
ELSE datediff(day,sdate,edate) END)
from a1 )
AS Feb_Days,
...
It's far from efficient. It will be more efficient to use a script or stored procedure running through your records and calculate the results.