DropdownList - Gridview - Show alert when no data available

Question

Hi,

   I have a dropdownlist contains Doctor's name.This names are brought from Database.Based on Doctor's name selection,the gridview contains Patients list will be generated from database.When the particular Doctor's name selected,the revelant patient names are shown in gridview.But if the particula Doctor doesn't have any patients then the page is blank.Now i've to show an alert about that particular doctor doesn't have any patients.Can anyone help me?