The Display Format of the External Data Control is set to drop down list, at runtime, the control renders blank values.
While creating all operations on the DB table during content type creation (or) double click on the Read List operation if you have already created all operations
1. Click on “Next”
2. Click on “Add Filter Parameter”
3. Click on Filter (Click to add)
4. Select Filter Type as “Wildcard”
5. Select a Filter Field and click OK
6. Click on “Finish” (Note you may need to change the “Data Source Element” to match the selected Filter Field)
7. Save content type.