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Ben Nadel
On User Experience (UX) Design, JavaScript, ColdFusion, Node.js, Life, and Love.

Ask Ben: Getting A Random Date From A Date Range In SQL

By Ben Nadel on
Tags: Ask Ben, SQL

How can I select a random date from a date range in SQL server. I basically want to do RandRange() for dates in sql? Does that make sense?

I have to admit, this one totally stumped me at first. I rarely work with random data in SQL. If I need random data I usually just build a query in ColdFusion. I tried looking up random methods for SQL, but it turns out that things like RAND() are only called once per table, NOT per row. I got some great ideas from Jon Galloway over at http://weblogs.asp.net/jgalloway/archive/2004/03/18/92498.aspx. He supplied the way to come up with a random date:

cast(
	cast( getdate() as int )
	-5 * rand( cast( cast( newid() as binary(8) ) as int ) )
	as datetime
	)

I am not exactly sure what the casting of the UUID to a binary(8) does, but I see that it converts to an integer.

After some experimentation, I see that casting UUIDs to binary can result in either a negative or positive number. For a date range, I needed to work with a positive number. Therefor I use ABS() to force a positive outcome. Once we have that, all we need is the date range part. That's where our old friend date-math comes in. You can think of a random date from a date range as adding a random number of days to the start date (>= 0) such that the resultant date does not exceed the end range date.

Once you think about it that way, we have all the pieces we need:

-- First, let's declare the date range. I am declaring this
-- here for the demo, but this could be done anyway you like.
DECLARE @date_from DATETIME;
DECLARE @date_to DATETIME;

-- Set the start and date dates. In this case, we are using
-- the month of october, 2006.
SET @date_from = '2006-10-01';
SET @date_to = '2006-10-30';

-- Select random dates.
SELECT
	(
		-- Remember, we want to add a random number to the
		-- start date. In SQL we can add days (as integers)
		-- to a date to increase the actually date/time
		-- object value.
		@date_from +
		(
			-- This will force our random number to be GTE 0.
			ABS(

				-- This will give us a HUGE random number that
				-- might be negative or positive.
				CAST(
					CAST( NewID() AS BINARY(8) )
					AS INT
				)
			)

			-- Our random number might be HUGE. We can't have
			-- exceed the date range that we are given.
			-- Therefore, we have to take the modulus of the
			-- date range difference. This will give us between
			-- zero and one less than the date range.
			%

			-- To get the number of days in the date range, we
			-- can simply substrate the start date from the
			-- end date. At this point though, we have to cast
			-- to INT as SQL will not make any automatic
			-- conversions for us.
			CAST(
				(@date_to - @date_from)
				AS INT
			)
		)
	)

Hope that helps.



Reader Comments

This works as well.

select DATEADD(second, rand()*86400, DATEADD(d, rand()*(CAST((@date_to - @date_from) AS INT )), @date_from))

86400, being the number of seconds in a day.

Cheers.

Thanks! This script helped me a lot in trying to generate a buncha test data where we're tracking stats based on unique datetimes.

Here's a slightly modified version... selecting a random time during an 8hr period during a particular day. (28800 is the number of seconds in 8 hours.) I send the NEWID() through RAND() to guarantee a unique timestamp for each row.

DECLARE @date_from DATETIME;
DECLARE @date_to DATETIME;

SET @date_from = '2009-06-29 08:00:00.000';
SET @date_to = '2009-06-29 16:00:00.000';

SELECT DATEADD(second, RAND(ABS(CAST(CAST(NewID() AS BINARY(8)) AS INT)))*28800, DATEADD(d, RAND(ABS(CAST(CAST(NewID() AS BINARY(8)) AS INT)))*(CAST((@date_to - @date_from) AS INT )), @date_from))

Very good code. It worked fine, the only thing that I have to check is the Date Format, as my SQL Server 2005 is in Portuguese I had to change the format to YYYY-DD-MM

Thank you

I did it like that

random date betweem 1-9-2009 to 29-1-2010
(between 0 to 150 days)

DECLARE @RandomNumber float
DECLARE @RandomDayF int
DECLARE @MaxDay int
DECLARE @MinDay int

SET @MaxDay = 150
SET @MinDay = 0

-- random the number of days to add
SELECT @RandomNumber = RAND()

set @randomDayF = ((@MaxDay + 1) - @MinDay) * @RandomNumber + @MinDay

-- add the days
Set @FromDate = (select DATEADD(d, @randomDayF,(SELECT convert(datetime, '1-9-2009',105)))) -- dd-mm-yyyy

--to reduce days
-- @randomDayF = @randomDayF * -1

I used this sample to generate random number from 1 to 1000000000
amazingly it produce 9-digit most of the time

I want it to generate more like 1,2,3,..9-digit

actually I need it in a lottery app
any other suggestion I'll appreciate it