对Linq查询感到困惑

Question

我创建了一个名为Hospital的简单数据库，我有几列。我已经填写了第一个名为drdoctors的下拉列表。它可以正常工作

 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();


    }

我想做的就是填写医生病人的另一份记录。

 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 


    }

但是linq查询非常复杂。我怎么能这样做

Answer 1

应该这样做。请注意，此代码未经过测试，可能包含轻微错误。

 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

查看图表，看起来您没有设置外键关系。我强烈建议这样做（出于无数原因）。但通过这样做，您将能够更加轻松地“加入”表格：

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();
}