Default value for database query if URL param blank

All,

I have a page that has a table with several columns one of which is Year (yyyy). I have a filter on the page that a user can select which years to show. This selection reloads the page and the URL has a comma list of the years that gets converted into an array for the DB query. But when the page loads initially there is no year selected. So, how can I force a default year value when the page loads for the first time.

Thanks.