Confused about Linq query - asp.net

Question

I have created a simple db named Hospital and I have a few columns .I have filled first dropdown named drdoctors.And it works
protected void Page_Load(object sender, EventArgs e)
{
if (!Page.IsPostBack) {
fetchDoctors();
}
}
void fetchDoctors() {
HospitalEntities entitiy = new HospitalEntities();
List<Doctor> doc = entitiy.Doctors.ToList();
drDoctor.DataSource = doc;
drDoctor.DataTextField = "Name";
drDoctor.DataValueField = "DoctorNo";
drDoctor.DataBind();
}
What I want to do is fill the other dropdown with the this doctor's patients .
protected void drDoctor_SelectedIndexChanged(object sender, EventArgs e)
{
int id= Int32.Parse( drDoctor.SelectedValue);
HospitalEntities entities = new HospitalEntities();
var query= from p in entities.Doctors
}
But linq queries are so complicated.How can i do this

Answer 1

This should about do it. Please note, this code wasn't tested and may contain minor errors.
protected void drDoctor_SelectedIndexChanged(object sender, EventArgs e)
{
int id= Int32.Parse( drDoctor.SelectedValue);
HospitalEntities entities = new HospitalEntities();
var query= (from d in entities.Doctors
join m in entities.MedExams on d.DoctorNo equals p.DoctorNo
join p in entities.Patients on m.PatientNo equals p.PatientNo
where d.DoctorNo == id
select p).ToList();
//Populate Patients from query
}

Answer 2

Looking at your diagram, it looks you don't have foreign key relationships set up. I would highly recommend doing this (for uncountable reasons). But by doing this, you will be able to "join" on tables much more easily like this:
protected void drDoctor_SelectedIndexChanged(object sender, EventArgs e)
{
int id = Int32.Parse(drDoctor.SelectedValue);
HospitalEntities entities = new HospitalEntities();
drDoctor.DataSource = entities.Doctors
.Where(x => x.DoctorNo == id)
.SelectMany(x => s.MedExams.Select(y => y.Patients));
drDoctor.DataBind();
}