Sql Server 2008 Calculate nights between two dates but remove 31st night - sql

Question

I have a booking table with the following information:
BookingID,(unique, not null)
StartDate, (not null)
EndDate (not null)
I need to calculate the number of nights someone remained in residence which I can do with a DATEDIFF between EndDate and StartDate. However, if someone is in residence for the entire month during a 31 day month we only charge them 30.
I'm not sure how to do this in SQL. I was thinking I would have to create a variable, calculate on a monthly basis and add to the variable, but that seems like it would be messy and take a very long time, especially towards the end of year. This needs to be done for about 5,000 records on a daily basis.
So:
If someone starts on 7/25/14 and ends 9/2/14, the nights is 38 not 39.
If someone starts on 10/2/14 and ends on 11/1/14, the nights is 30.
If someone starts on 10/2/14 and ends on 10/31/14, the nights is 29.
We will be calculating into the future so it doesn't matter if the end date is greater than the day the report is being ran.
Does anyone have any ideas how to accomplish this in the best way?

Answer 1

I would first to create a lookup table with all the month with 31 days
Such as
DECLARE #month TABLE (start_date date,end_date date)
INSERT INTO #month VALUES ('2014-07-01','2014-07-31'),('2014-08-01','2014-08-31'),('2014-10-01','2014-10-31'),('2014-12-01','2014-12-31')
//populate all months in your calculate range
Then you can calculate the value with
DECLARE #start DATE = '2014-07-25', #end DATE = '2014-09-02'
SELECT DATEDIFF(day,#start,#end) -
(SELECT COUNT(*) FROM #month WHERE start_date >= #start AND end_date <= #end)

Answer 2

Retrieve the integer part of the datediff divided by 31 :
SELECT DATEDIFF(day,'2014-07-25', '2014-09-02') - DATEDIFF(day,'2014-07-25', '2014-09-02') / 31

Answer 3

The SQLish solution is to create a calendar table that holds all the dates you will ever care about and any business meaning that those dates may have, such as "Is this a holiday?", "Is this off-season?", or "do we charge for this day?"
This may sound insane to someone accustomed to other programming languages, but it is perfectly sensible in the database world. Business rules and business logic get stored as data, not as code.
Make this table and populate it:
CREATE TABLE Calendar (
[date] date
,[is_charged] bit
)
and then you can write code that is nearly plain English:
SELECT
[BookingID]
,COUNT([date])
FROM BookingTable
INNER JOIN Calendar
ON [date] >= [StartDate]
AND [date] < [EndDate]
WHERE [is_charged] = 1
GROUP BY [BookingId]
When your business rules change, you just update the calendar instead of rewriting the code.

Answer 4

If I've read your question correctly then you can't actually use those solutions above which consist of a table of billable and non billable days because the 31st is billable unless the whole month was booked.
I reckon this is probably a job for a user defined function. Which runs up a total starting with the month that the start date is in and finishing with the month that the end date is in.
CREATE FUNCTION dbo.FN_BillableDays (#StartDate date, #EndDate date)
returns int
AS
BEGIN
IF #StartDate > #EndDate
BEGIN
return null --Error
END
DECLARE #Next date
DECLARE #MonthStart date
DECLARE #MonthEnd date
DECLARE #NextMonthStart date
DECLARE #n int =0
SET #Next = #StartDate
SET #MonthStart = DATEADD(day,1-DAY(#Next),#Next)
SET #NextMonthStart = DATEADD(month,1,#MonthStart )
SET #MonthEnd = DATEADD(day,-1,#NextMonthStart)
WHILE DATEDIFF(month,#Next,#EndDate) >0
BEGIN
SET #n = #n +
CASE
WHEN DAY(#next) = 1 AND DAY(#MonthEnd) = 31 THEN 30
WHEN DAY(#next) = 1 THEN DAY(#MonthEnd)
ELSE 1+DAY(#MonthEnd) -DAY(#next) END
SET #MonthStart = #NextMonthStart
SET #NextMonthStart = DATEADD(month,1,#MonthStart )
SET #MonthEnd = DATEADD(day,-1,#NextMonthStart)
SET #Next = #NextMonthStart
END
--Month of the EndDate
SET #n = #n +
CASE
WHEN DAY(#next) = 1 AND DAY(#EndDate) = 31 THEN 29
WHEN DAY(#next) = 1 THEN DAY(#EndDate)-1
ELSE DAY(#MonthEnd) -DAY(#EndDate) END
return #n
END
I tried it with some test dates
SELECT
b.BookingID,
b.StartDate,
b.EndDate,
dbo.FN_BillableDays (b.StartDate,b.EndDate) AS BillableDays
FROM dbo.Booking b
And got the following
BookingID StartDate EndDate BillableDays
----------- ---------- ---------- ------------
1 2013-12-31 2014-01-02 2
2 2013-12-31 2014-01-30 30
3 2014-01-01 2014-01-30 29
4 2014-01-01 2014-01-31 29
5 2014-01-01 2014-02-01 30
6 2014-01-01 2014-02-02 31
7 2014-02-02 2014-02-01 NULL
(7 row(s) affected)
Which matches my understanding of the logic you want to implement but you may want to tweak the last bit which adds on the days for the final month. If they leave on the 31st do you want to give them their last night (30th to 31st) for free.
If you don't then delete the line
WHEN DAY(#next) = 1 AND DAY(#EndDate) = 31 THEN 29