How to get values in textboxes after selection value of combox?

how can i get data in textboxes from database if my combobox selected values are from table1 and the values i want to get in textboxes are from table2 ?

private void Result_Load(object sender, EventArgs e)
       {

           string query = "SELECT  class_id , class_name FROM class";
           fillcombo(comboBox3, query, "class_name", "class_id");
          // comboBox3_SelectedIndexChanged(null, null);

       }
       public void fillcombo(ComboBox combo, string query, string displayMember, string valueMember)
       {
           command = new SqlCommand(query, con);
           adapter = new SqlDataAdapter(command);
           table = new DataTable();
           adapter.Fill(table);
           combo.DataSource = table;
           combo.DisplayMember = displayMember;
           combo.ValueMember = valueMember;
       }

private void comboBox3_SelectedIndexChanged(object sender, EventArgs e)
       {

           con.Open();
           int val;
           Int32.TryParse(comboBox3.SelectedValue.ToString(), out val);
           //string query = "SELECT  sub_name, class_id FROM subject  WHERE class_id = " + val;
           string query = "SELECT dbo.Subject.sub_name, dbo.Class.class_name FROM     dbo.Class full JOIN dbo.Subject ON dbo.Class.class_id = dbo.Subject.class_id where Class.class_id="+val;
           //string query = "SELECT sub_name FROM subject";


          SqlCommand command = new SqlCommand(query, con);

           using (var reader = command.ExecuteReader())
           {
               //SqlDataReader sdr = command.ExecuteReader();
               if (reader.Read())
               {


                   textBox1.Text = reader["sub_name"].ToString();
                   textBox2.Text = reader["sub_name"].ToString();
                   textBox3.Text = reader["sub_name"].ToString();
                   textBox4.Text = reader["sub_name"].ToString();
                   textBox5.Text = reader["sub_name"].ToString();
                   textBox7.Text = reader["sub_name"].ToString();
                   textBox8.Text = reader["sub_name"].ToString();
                   textBox9.Text = reader["sub_name"].ToString();
                   textBox10.Text = reader["sub_name"].ToString();
                   textBox11.Text = reader["sub_name"].ToString();




               }
               reader.Close();
           }
           con.Close();
       }