Question

I’ve created a sample data set in a MySql database that I need to use to test generating some reports with. For one particular table I’ve got 100k records that were all created within a few hours of each other. Because of the way the sample data was generated I couldn’t set the datetime columns on the 100k records to the desired values.

How can I use an SQL statement to randomise the datetime columns on the 100k records so instead of being within a few hours of each other they are spread across a month? As an example I’d like to spread the records evenly across the interval 1st Oct 2012 to 31st Oct 2012.

Answer 1

We can use RAND() function. Following MySQL Reference Manual, to obtain a random integer R in the range i <= R < j, use the expression FLOOR(i + RAND() * (j – i)). So for every row in your sample table, you’d need to run the following query:

SET @year = '2012';
SET @month = '10';
SET @day = FLOOR(1 + RAND() * (31 - 1));
SET @hour = FLOOR(0 + RAND() * (23 - 0));
SET @minute = FLOOR(0 + RAND() * (59 - 0));
SET @second = FLOOR(0 + RAND() * (59 - 0));

SET @date = CONCAT(@year,'-',@month,'-',@day,'-',@hour,'-',@minute,'-',@second);

UPDATE `sample_tbl`
   SET `date` = STR_TO_DATE(@date,'%Y-%c-%e-%k-%i-%s');