How to reuse query conditions?
You can use the function db_and() an db_or() to compile condition sets that you can the add to an existing query like this:

$query = db_select('Exhibitors', 'e');
$db_and = db_and();
$db_and-&gt;condition('ProductCategory', array(1,2,3), 'IN');
$db_and-&gt;condition('status', 'active');
$query-&gt;condition($db_and);


Example taken form here: https://api.drupal.org/api/drupal/includes!database!database.inc/function/db_and/7

Other than that, there is the DatabaseCondition Class that might be interesting for you too.