Introduction

In several cases in life we encounter a date range defined by its start and end dates, and we need to split the range into months included in that range. For example, we may need to find the months included within a given date range, or in a bit more complex scenario, we may need to find the exact date range in each of the months included within a given date range. In that case if our date range is defined by starting date 2014-03-14, and ending date of 2014-05-16 then our result set should be:

From Date       To Date
2014-03-14     2014-03-31
2014-04-01     2014-04-30
2014-05-01     2014-05-16

This short article shows how we can split a single date's range (start date, end date) into the data set of months which included in that range. In the next step we will use this solution to break a set of date ranges (table) into one set of months.

Our Case Study

We got a table with date ranges records, defined by its start and end dates. Each record includes a value for the date range, for example, it can be total number of hours we work on a project. Our goal is to split the working hours into months, relative to the number of days in that month included in the date range.

For example if we work totally of 20 hours from 2014-02-26 to 2014-03-02 then our original (source) data is:

Start Time        End Time        Working Hours
2014-02-26       2014-03-02     20

We worked 3 days in February and 2 days in March. Since we have total number of 20 then we will split the 20 hours into 5 days, so each day have average time of 4 working hours, therefore our result set will be:

Start Time        End Time        Days      Average Working Time This month
2014-02-26      2014-02-28    3             4*3 = 12
2014-03-01      2014-03-02    2             4*2 = 8

Let's show this in database language. This is our original (source) DDL+DML:

IF OBJECT_ID('dbo.MyTbl', 'U') IS NOT NULL DROP TABLE dbo.MyTbl
GO
 
-- I have a table which contains these columns - start date, end date and volumes
create table MyTbl (
      start_date datetime
    , end_date datetime
    , volumes int
)
GO
 
insert MyTbl
select '20140310','20140310',100 union all
select '20130310','20140310',244 union all
select '20120310','20120516',222 union all
select '20140210','20140212',456 union all
select '20140210','20140309',3333
GO
 
select * from MyTbl
GO

We can see that the first record includes only 1 day of working, therefore in the result set we will get 1 record. But the second record includes a full year of working, therefore this record should split into 12 months included in the date range, and each month should get number of average working hours relative to the numbers of days in that month. The third record includes 3 months... and so on...

Solution and Explanation

In our solution we are going to base on finding each starting date and ending date of the month. In order to do this we will use this logic (You can use different logic for this):

-- get first+last day in the month
declare @SDate datetime = '20130210'
SELECT
    DATEADD(mm, DATEDIFF(mm,0,@SDate), 0),
    DATEADD(DAY,-1,DATEADD(mm, DATEDIFF(mm,0,@SDate)+1, 0))
GO

We will start with a solution for splitting a single date range, which can be use for creating a function.

declare @SDate datetime = '20140310', @EDate datetime = '20140615'
select FirstDayOfMonth,LastDayOfMonth,DATEDIFF(DAY,FirstDayOfMonth,LastDayOfMonth)+1 [Number Of Days This month]
from (
    select top 100
        FirstDayOfMonth = CASE
            when DATEADD(MONTH, DATEDIFF(MONTH,0,DATEADD(MONTH,N,@SDate)), 0) < @SDate
                then @SDate
            else
                DATEADD(MONTH, DATEDIFF(MONTH,0,DATEADD(MONTH,N,@SDate)), 0)
         end
        , LastDayOfMonth = CASE
            when
            DATEADD(DAY,-1,DATEADD(mm, DATEDIFF(MONTH,0,DATEADD(MONTH,N,@SDate))+1, 0)) > @EDate
                then @EDate
            else
                DATEADD(DAY,-1,DATEADD(mm, DATEDIFF(MONTH,0,DATEADD(MONTH,N,@SDate))+1, 0))
         end
    from dbo.Numbers
    where
        DATEADD(mm, DATEDIFF(mm,0,DATEADD(MONTH,N,@SDate)), 0) < @EDate
        -- without this filter you will get error
        -- "Adding a value to a 'datetime' column caused an overflow"
        and N < 1000
) T
GO

Now let's move to a more complex solution without using a function for each record. In this solution we will use OUTER APPLY in order to work on the entire table as a SET. 

select start_date,end_date,volumes,FirstDayOfMonth,LastDayOfMonth
from MyTbl V
OUTER APPLY
(
    select top 1000 -- without this filter you will get error: "Adding a value to a 'datetime' column caused an overflow"
         FirstDayOfMonth = CASE
            when DATEADD(MONTH, DATEDIFF(MONTH,0,DATEADD(MONTH,N,V.start_date)), 0) < V.start_date
                then V.start_date
            else
                DATEADD(MONTH, DATEDIFF(MONTH,0,DATEADD(MONTH,N,V.start_date)), 0)
         end
        , LastDayOfMonth = CASE
            when
            DATEADD(DAY,-1,DATEADD(mm, DATEDIFF(MONTH,0,DATEADD(MONTH,N,V.start_date))+1, 0)) > V.end_date
                then V.end_date
            else
                DATEADD(DAY,-1,DATEADD(mm, DATEDIFF(MONTH,0,DATEADD(MONTH,N,V.start_date))+1, 0))
         end
    from dbo.Numbers
) T
where FirstDayOfMonth <= V.end_date

Next we need to use our splitting solution in order to get the Average working time for each month.

select
    start_date,
    end_date,
    volumes,
    FirstDayOfMonth,
    LastDayOfMonth,
    DATEDIFF(DAY,FirstDayOfMonth,LastDayOfMonth)+1 as NumberOfDaysThisMonth,
    DATEDIFF(DAY,start_date,end_date) + 1 TotalNumberOfDays,
    (  CONVERT(float,DATEDIFF(DAY,FirstDayOfMonth,LastDayOfMonth)+1)/(DATEDIFF(DAY,start_date,end_date) + 1)  ) * volumes as volumesPerMonth
from MyTbl V
OUTER APPLY
(
    select top 1000
         FirstDayOfMonth = CASE
            when DATEADD(MONTH, DATEDIFF(MONTH,0,DATEADD(MONTH,N,V.start_date)), 0) < V.start_date
                then V.start_date
            else
                DATEADD(MONTH, DATEDIFF(MONTH,0,DATEADD(MONTH,N,V.start_date)), 0)
         end
        , LastDayOfMonth = CASE
            when
            DATEADD(DAY,-1,DATEADD(mm, DATEDIFF(MONTH,0,DATEADD(MONTH,N,V.start_date))+1, 0)) > V.end_date
                then V.end_date
            else
                DATEADD(DAY,-1,DATEADD(mm, DATEDIFF(MONTH,0,DATEADD(MONTH,N,V.start_date))+1, 0))
         end
    from _ArielyAccessoriesDB.dbo.Numbers
) T
where FirstDayOfMonth <= V.end_date

Summary

In this article we tried to show how we can split date ranges into a set of monthly records.

Comments

  • Numbers Table:

    • The code used numbers table dbo.Numbers
    • This is highly recommended to have a numbers table in the database, or in general read only accessories database. This table is indexed using clustered index and will give us better solution then build it on-the-fly in each query. If you don't have one, please build a number table.

Resources

* This article is based on Ronen Ariely's code posted in the blog:
   http://ariely.info/Blog/tabid/83/EntryId/141/Split-Date-Range-into-Months.aspx

Forums Questions

The topic covered in this article is quite common, and there are dozens of questions in various forums related to it.