On the change event of drop-down you need to again bind your table, with fresh data.
Step 1: Get selected drop-down value. (ByValue or ByIndex)
Step 2: Select query with Where condition on selected field of drop-down.
Step 3: Bind new internal table to webdynpro table.
Similar example, WebDynpro for ABAP - Scenario on displaying listbox (Multiple selections)
Hope this helps.