Связь один ко многим

        public Form1()
        {
            InitializeComponent();
            this.Size = new Size(500, 500);
            connection = new SqlConnection(@"Data Source=COMP\SQLEXPRESS;Initial Catalog=cyber;Integrated Security=True");
            adapterGroups = new SqlDataAdapter("SELECT id, name FROM groups;", connection);
            adapterStudents = new SqlDataAdapter("SELECT id, fio, idGroup FROM students", connection);
            dataSet = new DataSet();
            adapterGroups.Fill(dataSet, "groups");
            adapterStudents.Fill(dataSet, "students");
            dataSet.Relations.Add("groups-students", //название связи
                dataSet.Tables["groups"].Columns["id"],//первичный ключ главной таблицы
                dataSet.Tables["students"].Columns["idGroup"]);//внешний ключ подчиненной таблицы
            bindingSourceGroups = new BindingSource(dataSet, "groups");
            bindingSourceStudents = new BindingSource(bindingSourceGroups, "groups-students");
            gridGroups = new DataGridView();
            gridGroups.Size = new Size(this.ClientRectangle.Width - 20, (this.ClientRectangle.Height >> 1) - 15);
            gridGroups.Location = new Point(10, 10);
            gridGroups.DataSource = bindingSourceGroups;
            gridStudents = new DataGridView();
            gridStudents.Size = gridGroups.Size;
            gridStudents.Location = new Point(10, gridGroups.Bottom + 10);
            gridStudents.DataSource = bindingSourceStudents;
            this.Controls.AddRange(new Control[] { gridGroups, gridStudents });
        }

private void Form1_Load(object sender, EventArgs e)
{
    using (OleDbConnection conn = new OleDbConnection(connString))
    {
        string commText = "SELECT * FROM Dept";
        OleDbCommand comm = new OleDbCommand(commText, conn);
        DataTable table = new DataTable();
        OleDbDataAdapter adapter = new OleDbDataAdapter(comm);
        adapter.Fill(table);
        comboBox1.DataSource = table;
        comboBox1.DisplayMember = "Dept_Name"; //отображаемое поле
        comboBox1.ValueMember = "ID_Dept"; //поле-значение
        comboBox1.SelectedIndex = -1; //снимаем выбор
    }
    comboBox1.SelectedIndexChanged += new EventHandler(comboBox1_SelectedIndexChanged);
}
 
private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
    using (OleDbConnection conn = new OleDbConnection(connString))
    {
        string commText = "SELECT * FROM Printer where ID_Dept = @ID_dept";
        OleDbCommand comm = new OleDbCommand(commText, conn);
        comm.Parameters.AddWithValue("@ID_dept", (int)(sender as ComboBox).SelectedValue);
        DataTable table = new DataTable();
        OleDbDataAdapter adapter = new OleDbDataAdapter(comm);
        adapter.Fill(table);
        listBox1.DataSource = table;
        listBox1.DisplayMember = "Printer_Name"; //отображаемое поле
        listBox1.ValueMember = "ID_Print"; //поле-значение
        listBox1.SelectedIndex = -1; //снимаем выбор
    }
}