MySQL, VB.NET สอบถามปัญหาการทำ multiple combobox ให้สัมพันธ์กัน

 Private Sub frmPeople_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        lblPeopleID.BackColor = Color.FromArgb(0, 0, 0, 0)

        Dim FD As New FormFader
        FD.FadeIn(Me, 0.25)

        sw.Reset()
        sw.Start()
        Dim strConn As String
        strConn = DBConnString.strConn

        Conn = New SqlConnection()
        With Conn
            If .State = ConnectionState.Open Then .Close()
            .ConnectionString = strConn
            .Open()
        End With

        sb = New StringBuilder()
        sb.Append("SELECT * FROM tbl_preT;")
        sb.Append("SELECT * FROM tbl_preE;")
        sb.Append("SELECT * FROM tbl_type;")
        sb.Append("SELECT * FROM tbl_campus;")
        sb.Append("SELECT * FROM tbl_dep1 ORDER BY dep1Name;")
        sb.Append("SELECT * FROM tbl_dep2 ORDER BY dep2Name;")

        Dim sqlIni As String
        sqlIni = sb.ToString()

        RemoveHandler cboPreT.SelectedValueChanged, AddressOf cboPreT_SelectedValueChanged
        RemoveHandler cboPreE.SelectedValueChanged, AddressOf cboPreE_SelectedValueChanged
        RemoveHandler cboCampus.SelectedValueChanged, AddressOf cboCampus_SelectedValueChanged
        RemoveHandler cboDep1.SelectedValueChanged, AddressOf cboDep1_SelectedValueChanged
        '    RemoveHandler cboDep2.SelectedValueChanged, AddressOf cboDep2_SelectedValueChanged
        RemoveHandler cboType.SelectedValueChanged, AddressOf cboType_SelectedValueChanged

        com = New SqlCommand()
        With com
            .CommandText = sqlIni
            .CommandType = CommandType.Text
            .Connection = Conn
            dr = .ExecuteReader()
            If dr.HasRows Then

                dtPreT = New DataTable
                dtPreT.Load(dr)
                With cboPreT
                    .BeginUpdate()
                    .DisplayMember = "preTNameS"
                    .ValueMember = "preTID"
                    .DataSource = dtPreT
                    .EndUpdate()
                End With

                dtPreE = New DataTable
                dtPreE.Load(dr)
                With cboPreE
                    .BeginUpdate()
                    .DisplayMember = "preENameS"
                    .ValueMember = "preEID"
                    .DataSource = dtPreE
                    .EndUpdate()
                End With

                dtType = New DataTable
                dtType.Load(dr)
                With cboType
                    .BeginUpdate()
                    .DisplayMember = "typeName"
                    .ValueMember = "typeID"
                    .DataSource = dtType
                    .EndUpdate()
                End With

                dtCampus = New DataTable
                dtCampus.Load(dr)
                With cboCampus
                    .BeginUpdate()
                    .DisplayMember = "CampusNameS"
                    .ValueMember = "campusID"
                    .DataSource = dtCampus
                    .EndUpdate()
                End With

                dtDep1 = New DataTable
                dtDep1.Load(dr)
                With cboDep1
                    .BeginUpdate()
                    .DisplayMember = "dep1Name"
                    .ValueMember = "dep1ID"
                    .DataSource = dtDep1
                    .EndUpdate()
                End With

                dtDep2 = New DataTable
                dtDep2.Load(dr)
                With cboDep2
                    .BeginUpdate()
                    .DisplayMember = "dep2Name"
                    .ValueMember = "dep2ID"
                    .DataSource = dtDep2
                    .EndUpdate()
                End With

            End If
        End With
        dr.Close()

        AddHandler cboPreT.SelectedValueChanged, AddressOf cboPreT_SelectedValueChanged
        AddHandler cboPreE.SelectedValueChanged, AddressOf cboPreE_SelectedValueChanged
        AddHandler cboCampus.SelectedValueChanged, AddressOf cboCampus_SelectedValueChanged
        AddHandler cboDep1.SelectedValueChanged, AddressOf cboDep1_SelectedValueChanged
        '   AddHandler cboDep2.SelectedValueChanged, AddressOf cboDep2_SelectedValueChanged
        AddHandler cboType.SelectedValueChanged, AddressOf cboType_SelectedValueChanged

        iPreT.SelectedValue = cboPreT.SelectedValue
        iPreE.SelectedValue = cboPreE.SelectedValue
        iCampus.SelectedValue = cboCampus.SelectedValue
        iDep1.SelectedValue = cboDep1.SelectedValue
        iDep2.SelectedValue = cboDep2.SelectedValue
        iType.SelectedValue = cboType.SelectedValue

        With cboSearch
            .Items.Add("รหัสบุคคล")
            .Items.Add("ชื่อบุคคล(ภาษาไทย)")
            .Items.Add("ชื่อบุคคล(ภาษาอังกฤษ)")
            '  .Items.Add("คณะ")
            '  .Items.Add("ภาควิชา")
            .Items.Add("เบอร์โทรศัพท์มือถือ")
            .SelectedIndex = 1
        End With

        cboPreT.ContextMenu = New ContextMenu()
        cboPreE.ContextMenu = New ContextMenu()
        cboCampus.ContextMenu = New ContextMenu()
        cboDep1.ContextMenu = New ContextMenu()
        cboDep2.ContextMenu = New ContextMenu()
        cboType.ContextMenu = New ContextMenu()

        picPeople.Image = picPeople.ErrorImage
        ' dtpAdmitDate.Value = DateTime.Today()
        ClearCustomer()

        tsAdd.Enabled = True
        tsOK.Enabled = False
        '  tsCancel.Enabled = False

        sw.Stop()
        ProcessTime = sw.ElapsedMilliseconds / 1000
        _ToolStripLabel.Text = "เวลาที่ใช้ : " & ProcessTime.ToString("0.0000") & "วินาที"
        If (iSubjectDetail = False) Then
            bgwPeople.RunWorkerAsync()
        End If
    End Sub

    Private Sub cboDep1_SelectedValueChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cboDep1.SelectedValueChanged
        iDep1.SelectedValue = cboDep1.SelectedValue
    End Sub

    Private Sub cboDep2_SelectedValueChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cboDep2.SelectedValueChanged
        iDep2.SelectedValue = cboDep2.SelectedValue
    End Sub