В зависимости от того, что выбрано в одном Combobox в другом на выборку давалось другое

Function strDBconnect(ByVal user As String, ByVal pwd As String, ByVal dbserver As String, ByVal dbname As String) As String
        Dim Server As String = dbserver
        Dim Database As String = dbname
        Dim UserID As String = user
        Dim Password As String = pwd
        Dim MyConString As String = "Server=" + Server + ";Database=" + Database + ";User ID=" + UserID + ";Password=" + Password
 
        Return MyConString
End Function

Function GetDB(ByVal myConnectionString As String, ByVal mySelectQuery As String) As DataTable
        Dim dt As New DataTable
        Dim myConnection As New MySqlConnection(myConnectionString)
        Dim myAdapter As New MySqlDataAdapter(mySelectQuery, myConnection)
 
        myConnection.Open()
        Try
            dt.Clear()
            myAdapter.Fill(dt)
        Catch ex As Exception
            MessageBox.Show(ex.Message)
        Finally
            myConnection.Close()
        End Try
 
        Return dt
End Function

Private Sub frm_main_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        Dim sql_conn As String = strDBconnect("user", "psw", "localhost", "db")
        Dim sql_query As String = "SELECT id, lastname FROM users WHERE type='Group'"
 
        cmb_department.ValueMember = "id"
        cmb_department.DisplayMember = "lastname"
        cmb_department.DataSource = GetDB(sql_conn, sql_query)     
End Sub

Private Sub cmb_department_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmb_department.SelectedIndexChanged
        Dim sql_conn As String = strDBconnect("user", "psw", "localhost", "db")
        Dim sql_query As String = "SELECT users.lastname, users.id " _
                                      & "FROM users LEFT JOIN groups_users ON users.id = groups_users.user_id " _
                                     & "WHERE groups_users.group_id=" + Me.cmb_department.SelectedValue.ToString
 
        Me.cmb_manager.DisplayMember = "lastname"
        Me.cmb_manager.ValueMember = "id"
        Me.cmb_manager.DataSource = GetDB(sql_conn, sql_query)
End Sub