I can populate the form fields using JSON

I select a client from dropdown

and based on its value (id) i call JSON and populate the respective fields

 

what if the the client id  selected from dropdown is zero (normally starting item with id=0 like pleases select) )

  

My Json

where to put the initialization code so as to blank the fields populated earlier 

2. $('#clientid').change( function() { //change ID when selected from dropdown 
3. alert(   $('#clientid').val()); // correct value of client id is displayed 
4. $.getJSON("clientreadjsonfill.php", {id: $('#clientid').val()}, function(data){
5.                         $('#buildingname').val(data['buildingname']); // populate data  working
6. $('#roomno').val(data['roomno']);
7. $('#streetname').val(data['streetname']);
8. $('#location').val(data['location']);
9. $('#landmark').val(data['landmark']);
10. $('city').val(data['city']);
11. }).done(function(){
12. $('#aaa').text('xxx'); // xxx is populated in div with id aaa
13.                  
14.       });
15. });