how clear dropdwonlist if searh is null and how to get again

Question

Hi Guys

 

Please help me

 

I need to clear dropdownlists if the search is null like any value in dropdownlist is equal zero or null and how to get data again when the search is already found (DataBind). I search by all dropdownlists  in table above except Fleet_no. 

 

sepecific dropdownlists work as following :

 

1. driver name is control id for all dropdownlists in same row style

2. Custodian 1 name is control id for all dropdownlists in same row style

3. Custodian 2 name is control id for all dropdownlists in same row style.

 

Thanks