Parametric search tool looks for datasheets that exactly match your search criteria. Untouched fields will not affect the query. If you finished entering parameters click on the "Start Search" button to start the query. Then the form moves to left (the search criteria may be refined by you any time) and results are located on the right. You can also easily compare selected results after the query by using "Add to compare" buttons.
You are using the standard, simplified version of parametric search tool. To execute in-depth queries switch to detailed search tool: Detailed search tool