Hi,
This is the basic set up you'll need.
- One data connection that executes on load and populates the dropdown (returns only whatever data is needed for the dropdown).
- Rules on the dropdown to: (a) set the value of the query fields and (b) query a second data connection (which returns the columns you need)
- Then you can display the results of this second data connection.
Now, when configuring the dropdown, remember that dropdowns have a DisplayName and a Value. Value is usually the ID column of the SharePoint list, and this will be needed for the second data connection, in order to complete the query correctly.
Hope this helps. Good luck!