Select Multiple Rows from Timespan - sql

Question

Problem
In my sql-server-2014 I store projects in a table with the columns:
Startdate .. | Enddate ....| Projectname .................| Volume
2017-02-13 | 2017-04-12 | GenerateRevenue .........| 20.02
2017-04-02 | 2018-01-01 | BuildRevenueGenerator | 300.044
2017-05-23 | 2018-03-19 | HarvestRevenue ............| 434.009
I need a SELECT to give me one row per month of the project for each project. the days of the month don't have to be considered.
Date .......... | Projectname..................| Volume
2017-02-01 | GenerateRevenue .........| 20.02
2017-03-01 | GenerateRevenue .........| 20.02
2017-04-01 | GenerateRevenue .........| 20.02
2017-04-01 | BuildRevenueGenerator | 300.044
2017-05-01 | BuildRevenueGenerator | 300.044
2017-06-01 | BuildRevenueGenerator | 300.044
...
Extra
Ideally the logic of the SELECT allows me both to calculate the monthly volume and also the difference between each month and the previous.
Date .......... | Projectname..................| VolumeMonthly
2017-02-01 | GenerateRevenue .........| 6.6733
2017-03-01 | GenerateRevenue .........| 6.6733
2017-04-01 | GenerateRevenue .........| 6.6733
2017-04-01 | BuildRevenueGenerator | 30.0044
2017-05-01 | BuildRevenueGenerator | 30.0044
2017-06-01 | BuildRevenueGenerator | 30.0044
...
Also...
I know I can map it on a temporary calendar table, but that tends to get bloated and complex very fast. Im really looking for a better way to solve this problem.
Solution
Gordons solution worked very nicely and it doesn't require a second table or mapping on a calendar of some sort. Although I had to change a few things, like making sure both sides of the union have the same SELECT.
Here my adapted version:
with cte as (
select startdate as mondate, enddate, projectName, volume
from projects
union all
select dateadd(month, 1, mondate), enddate, projectName, volume
from cte
where eomonth(dateadd(month, 1, mondate)) <= eomonth(enddate)
)
select * from cte;
Volume monthly can be achieved by replacing volume with:
CAST(Cast(volume AS DECIMAL) / Cast(Datediff(month,
startdate,enddate)+ 1 AS DECIMAL) AS DECIMAL(15, 2))
END AS [volumeMonthly]

Answer 1

Another option is with an ad-hoc tally table
Example
-- Some Sample Data
Declare #YourTable table (StartDate date,EndDate date,ProjectName varchar(50), Volume float)
Insert Into #YourTable values
('2017-03-15','2017-07-25','Project X',25)
,('2017-04-01','2017-06-30','Project Y',50)
-- Set Your Desired Date Range
Declare #Date1 date = '2017-01-01'
Declare #Date2 date = '2017-12-31'
Select Period = D
,B.*
,MonthlyVolume = sum(Volume) over (Partition By convert(varchar(6),D,112))
From (Select Top (DateDiff(MONTH,#Date1,#Date2)+1) D=DateAdd(MONTH,-1+Row_Number() Over (Order By (Select Null)),#Date1)
From master..spt_values n1
) A
Join #YourTable B on convert(varchar(6),D,112) between convert(varchar(6),StartDate,112) and convert(varchar(6),EndDate,112)
Order by Period,ProjectName
Returns
Note: Use a LEFT JOIN to see gaps

Answer 2

You can use a recursive subquery to expand the rows for each project, based on the table:
with cte as (
select stardate as mondate, p.*
from projects
union all
select dateadd(month, 1, mondate), . . . -- whatever columns you want here
from cte
where eomonth(dateadd(month, 1, mondate)) <= eomonth(enddate)
)
select *
from cte;
I'm not sure if this actually answers your question. When I first read the question, I figured the table had one row per project.

Answer 3

Using a couple of common table expressions, an adhoc calendar table for months and lag() (SQL Server 2012+) for the final delta calculation:
create table projects (id int identity(1,1), StartDate date, EndDate date, ProjectName varchar(32), Volume float);
insert into projects values ('20170101','20170330','SO Q1',240),('20170214','20170601','EX Q2',120)
declare #StartDate date = '20170101'
, #EndDate date = '20170731';
;with Months as (
select top (datediff(month,#startdate,#enddate)+1)
MonthStart = 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
)
, ProjectMonthlyVolume as (
select p.*
, MonthlyVolume = Volume/(datediff(month,p.StartDate,p.EndDate)+1)
, m.MonthStart
from Months m
left join Projects p
on p.EndDate >= m.MonthStart
and p.StartDate <= m.MonthEnd
)
select
MonthStart = convert(char(7),MonthStart,120)
, MonthlyVolume = isnull(sum(MonthlyVolume),0)
, Delta = isnull(sum(MonthlyVolume),0) - lag(Sum(MonthlyVolume)) over (order by MonthStart)
from ProjectMonthlyVolume pmv
group by MonthStart
rextester demo: http://rextester.com/DZL54787
returns:
+------------+---------------+-------+
| MonthStart | MonthlyVolume | Delta |
+------------+---------------+-------+
| 2017-01 | 80 | NULL |
| 2017-02 | 104 | 24 |
| 2017-03 | 104 | 0 |
| 2017-04 | 24 | -80 |
| 2017-05 | 24 | 0 |
| 2017-06 | 24 | 0 |
| 2017-07 | 0 | -24 |
+------------+---------------+-------+