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מרץ31

Written by: ronen ariely
31/03/2014 02:21 RssIcon

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 include with in a given date range, or in a bit more complex demand, we may need to find the exact date range in each of the months include with in a given date range. In this 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 we will show 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 include 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, therefor 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 EXISTS(select OBJECT_ID('dbo.MyTbl', 'U')) DROP TABLE dbo.MyTbl
GO
  
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','20140310',222 union all
select '20140210','20140210',456 union all
select '20140210','20140310',3333
GO
  
select * from MyTbl
GO

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

Solutions and Explanation

In our solution we are going to based 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 _ArielyAccessoriesDB.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 SET.

select start_date,end_date,volumes,FirstDayOfMonth,LastDayOfMonth
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

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 show how we can split a date range into a set of monthly records.

Comments

  • The code used numbers table _ArielyAccessoriesDB.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.
  • Guy Glantser, commented and remind us that using SQL Server version 2012/2014 we can use the function EOMONTH, Thanks.
  • SaidaKishore Potla, commented that we can use case with between operator, Thanks
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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 

Forum's Question

Those needs we cover in this article are very common, and there are dozens of questions in various forums on topics related to it.