Question

What’s the easiest way to update a table that contains a DATETIME column on TSQL with RANDOM value between 2 dates?

I see various post related to that but their Random values are really sequential when you ORDER BY DATE after the update.

Answer 1

Assumptions

First assume that you have a database containing a table with a start datetime column and a end datetime column, which together define a datetime range:

CREATE DATABASE StackOverflow11387226;
GO

USE StackOverflow11387226;
GO

CREATE TABLE DateTimeRanges (
  StartDateTime DATETIME NOT NULL,
  EndDateTime DATETIME NOT NULL
);
GO

ALTER TABLE DateTimeRanges
ADD CONSTRAINT CK_PositiveRange CHECK (EndDateTime > StartDateTime);

And assume that the table contains some data:

INSERT INTO DateTimeRanges (
  StartDateTime,
  EndDateTime
)
VALUES
  ('2012-07-09 00:30', '2012-07-09 01:30'),
  ('2012-01-01 00:00', '2013-01-01 00:00'),
  ('1988-07-25 22:30', '2012-07-09 00:30');
GO

Method

The following SELECT statement returns the start datetime, the end datetime, and a pseudorandom datetime with minute precision greater than or equal to the start datetime and less than the second datetime:

SELECT
  StartDateTime,
  EndDateTime,
  DATEADD(
    MINUTE,
    ABS(CHECKSUM(NEWID())) % DATEDIFF(MINUTE, StartDateTime, EndDateTime) + DATEDIFF(MINUTE, 0, StartDateTime),
    0
  ) AS RandomDateTime
FROM DateTimeRanges;

Result

Because the NEWID() function is nondeterministic, this will return a different result set for every execution. Here is the result set I generated just now:

StartDateTime           EndDateTime             RandomDateTime
----------------------- ----------------------- -----------------------
2012-07-09 00:30:00.000 2012-07-09 01:30:00.000 2012-07-09 00:44:00.000
2012-01-01 00:00:00.000 2013-01-01 00:00:00.000 2012-09-08 20:41:00.000
1988-07-25 22:30:00.000 2012-07-09 00:30:00.000 1996-01-05 23:48:00.000

All the values in the column RandomDateTime lie between the values in columns StartDateTime and EndDateTime.

Explanation

This technique for generating random values is due to Jeff Moden. He wrote a great article on SQL Server Central about data generation. Read it for a more thorough explanation. Registration is required, but it’s well worth it.

The idea is to generate a random offset from the start datetime, and add the offset to the start datetime to get a new datetime in between the start datetime and the end datetime.

The expression DATEDIFF(MINUTE, StartDateTime, EndDateTime) represents the total number of minutes between the start datetime and the end datetime. The offset must be less than or equal to this value.

The expression ABS(CHECKSUM(NEWID())) generates an independent random positive integer for every row. The expression can have any value from 0 to 2,147,483,647. This expression mod the first expression gives a valid offset in minutes.

The epxression DATEDIFF(MINUTE, 0, StartDateTime) represents the total number of minutes between the start datetime and a reference datetime of 0, which is shorthand for '1900-01-01 00:00:00.000'. The value of the reference datetime does not matter, but it matters that the same reference date is used in the whole expression. Add this to the offset to get the total number of minutes between the reference datetime.

The ecapsulating DATEADD function converts this to a datetime value by adding the number of minutes produced by the previous expression to the reference datetime.