I'm trying to get an edit-combo list to display only unique values (similar to a Distinct clause in SQL).

Presently my setup is a List control based on a SQL query.
Two Edit-Combo controls are set to filter and restrict the users' choices.
The second Edit-Combo is dynamically filtered based on the choice in the first Edit-Combo.

Problem is how do I restrict the second Edit-Combo choices to Distinct or unique values? ie) in the example, I only want the user to see one 'Node' value and one 'SubNode' value, etc.

Filter Dropdown.jpg