How to validate and compare values on one record with other records in a multi record block?

  Hi 

I have seen posts on the forums that explain how to loop thro multiple records and perform validation  in a tabular database block .  My problem is a little more than just looping thro the records.

Let us say we have the following columns in the multiple record database block -

Product   Price Start_date  End_date

Users enter/update information in the rows under these columns and there are multiple rows. Once all changes have been made, the requirement is that before applying changes to the database, there should be a check performed to make sure that any one product does not have more than one entry for a specified time interval. As an example if I enter