Show all item in second Cascading DropDownList when default Item in first DropDownList

hi 

 i have 2 dropdownlist in my page

1-ddlzone1

2-ddlstore1

What I need is if I select default item i.e. 0 in Zone dropdown I need to show all items in store dropdown. Below is my stored procedure to get the stores

ALTER procedure [dbo].[selectcenter]
@RegionID varchar(5)
as
begin
select ID,Centername
from Shoppingcenter
where RegionID=@RegionID
group by ID,Centername
end