ขอคำแนะนำ เรื่อง การตัดจำนวนสินค้าเมื่อการสั่งซื้อหน่อยครับ

Imports System.Data
Imports System.Data.SqlClient




Public Class frmpurchase
    Dim connect As New SqlConnection
    Dim da As New SqlDataAdapter
    Dim ds As New DataSet


    Private Sub btncus_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btncus.Click
        Dim searchcustomer As New searchcustomer
        searchcustomer.MdiParent = frmmain
        searchcustomer.Show()
        Me.Hide()
    End Sub

    Private Sub btnemp_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnemp.Click
        Dim sqlshowemp As String

        Dim empid As String

        empid = txtempid.Text

        connect.ConnectionString = "Data Source=(local);Initial Catalog=udomdate;Integrated Security=True"
        connect.Open()

        sqlshowemp = "select * from employee where empid='" & empid & "'"

        da = New SqlDataAdapter(sqlshowemp, connect)
        da.Fill(ds, "emp")
        txtempname.Text = ds.Tables("emp").Rows(0)("empname").ToString
        ds.Clear()

        connect.Close()



    End Sub



    Private Sub frmpurchase_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        Dim sqlshowproduct As String
        Dim i As Integer ' i มาจาก interval คือการวนลูบข้อมูล รหัสสินค้าสู่ combobox

        connect.ConnectionString = "Data Source=(local);Initial Catalog=udomdate;Integrated Security=True"
        connect.Open()
        sqlshowproduct = "select * from product"

        da = New SqlDataAdapter(sqlshowproduct, connect)

        da.Fill(ds, "pro")

        For i = 0 To ds.Tables("pro").Rows.Count - 1
            cboproid.Items.Add(ds.Tables("pro").Rows(i)("proid").ToString)

        Next
        connect.Close()



    End Sub


    Private Sub cboproid_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cboproid.SelectedIndexChanged
        Dim sqlshowproduct As String
        Dim proid As String
        proid = cboproid.SelectedItem


        connect.ConnectionString = "Data Source=(local);Initial Catalog=udomdate;Integrated Security=True"
        connect.Open()

        sqlshowproduct = "select * from product where proid='" & proid & "'"

        da = New SqlDataAdapter(sqlshowproduct, connect)

        da.Fill(ds, "pro1")

        txtproname.Text = ds.Tables("pro1").Rows(0)("proname").ToString
        txtproprice.Text = ds.Tables("pro1").Rows(0)("proprice").ToString
        txtprostock.Text = ds.Tables("pro1").Rows(0)("prostock").ToString
        ds.Clear()



        connect.Close()
    End Sub

    Private Sub btnadd_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnadd.Click
        'ประกาศตัวแปรที่เกี่ยวข้อง
        Dim proid, proname As String
        Dim proprice, prosum As Double 'prosum คือตัวแปรรับผลรวมจาก จำนวน * ราคา
        Dim pronum As Integer
        ' เนื่องจากต้องตรวจสอบรายการที่ซื้อด้วย จะต้องใช้ วนลูป จึงต้องประกาศตัวแปร
        Dim i As Integer
        'ในการเก็บข้อมูลแต่ละรายการสู่ listview จะต้องกระทำพร้อมกัน โดยผ่านตัวแปร listview
        'ดังนั้นเพื่อให้สะดวกในการจัดเก็บข้อมูลสู่ listview จึงควรนำค่าที่ได้จาก combobox และ textbox
        'ใส่ตัวแปร array
        Dim proitem(5) As String
        Dim lvpro As New ListViewItem
        ' ตรวจสอบว่ามีการป้อนข้อมูลไว้ใน textbox จำนสน เรียบร้อย
        If txtnum.Text = "" Then
            MessageBox.Show("กรุณาป้อนจำนวน")
            txtnum.Focus()

            Exit Sub

        Else
            If txtnum.text <= 0 Then
                MessageBox.Show("กรุณาป้อนจำนวนซื้อที่มากกว่า 0")
                txtnum.clear()
                txtnum.focus()
                Exit Sub
            Else

                proid = cboproid.SelectedItem
                pronum = txtnum.Text

                For i = 0 To lsvbuy.Items.Count - 1

                    If lsvbuy.Items(i).SubItems(0).Text = proid Then
                        If (MessageBox.Show("คุณต้องการซื้อเพิ่มใช่หรือไม่", "ยินยันการซื้อ", MessageBoxButtons.YesNo, MessageBoxIcon.Question) = Windows.Forms.DialogResult.Yes) Then


                            pronum = pronum + lsvbuy.Items(i).SubItems(3).Text
                            lsvbuy.Items(i).Remove()
                            Exit For
                        Else 'ถ้าเลือก no คือไม่เพิ่มจำนวน
                            Exit Sub

                        End If

                    End If

                Next
                proname = txtproname.Text
                proprice = txtproprice.Text
                prosum = pronum * proprice
                proitem(0) = proid
                proitem(1) = proname
                proitem(2) = proprice
                proitem(3) = pronum
                proitem(4) = prosum

                lvpro = New ListViewItem(proitem)
                lsvbuy.Items.Add(lvpro)


            End If

        End If






    End Sub

    Private Sub btndel_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btndel.Click
        Dim itemdel As Object
        For Each itemdel In lsvbuy.CheckedItems
            If (MessageBox.Show("จะลบจริงๆหรอ", "ลบให้แล้วนะ", MessageBoxButtons.YesNo, MessageBoxIcon.Question) = Windows.Forms.DialogResult.Yes) Then

                lsvbuy.Items.Remove(itemdel)

            End If
        Next





    End Sub

    Private Sub btnsave_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnsave.Click
        Dim cusid As String
        Dim empid As String
        Dim proid As String
        Dim proprice As Double
        Dim pronum As Integer
        Dim prosum As Double
        Dim purtotal As Double
        Dim purdate As Date
        Dim purid As Integer


        Dim i As Integer
        Dim sqlcheckorder As String
        Dim sqladdorder As String
        Dim sqladddetail As String
        Dim cmd As New SqlCommand
        Dim cmd1 As New SqlCommand

        If txtcusid.Text = "" Then
            MessageBox.Show("กรุณาเลือกลูกค้า")
        Else
            If txtempid.Text = "" Then
                MessageBox.Show("กรุณาระบุพนักงานขาย")
            Else

                cusid = txtcusid.Text
                empid = txtempid.Text

                If lsvbuy.Items.Count <> 0 Then

                    For i = 0 To lsvbuy.Items.Count - 1

                        purtotal = purtotal + lsvbuy.Items(i).SubItems(4).Text

                    Next
                End If

                purdate = Today

                connect.ConnectionString = "Data Source=(local);Initial Catalog = udomdate;Integrated Security=true"
                connect.Open()

                sqladdorder = "insert into purchase(cusid,empid,purdate,purtotal) values('" & cusid & "','" & empid & "'," & purdate & "," & purtotal & ")"

                cmd.CommandType = CommandType.Text
                cmd.CommandText = sqladdorder
                cmd.Connection = connect
                cmd.ExecuteNonQuery()

                sqlcheckorder = "select max(purid) from purchase"

                da = New SqlDataAdapter(sqlcheckorder, connect)
                da.Fill(ds, "pur")
                purid = ds.Tables("pur").Rows(0).Item(0).ToString

                For i = 0 To lsvbuy.Items.Count - 1
                    proid = lsvbuy.Items(i).SubItems(0).Text
                    proprice = lsvbuy.Items(i).SubItems(2).Text
                    pronum = lsvbuy.Items(i).SubItems(3).Text
                    prosum = lsvbuy.Items(i).SubItems(4).Text
                    sqladddetail = "insert into detail(purid,proid,proprice,pronum,prosum) values(" & purid & ",'" & proid & "'," & proprice & "," & pronum & "," & prosum & " ) "

                    cmd.CommandType = CommandType.Text
                    cmd.CommandText = sqladddetail
                    cmd.Connection = connect
                    cmd.ExecuteNonQuery()


                Next

                MessageBox.Show("การสั่งซื้อสินค้าเรียบร้อย")
                ds.Clear()
                connect.Close()

                txtcusid.Clear()
                txtcusname.Clear()
                txtcusaddress.Clear()
                txtcustel.Clear()
                txtempname.Clear()
                cboproid.SelectedItem = ""
                txtproname.Clear()
                txtproprice.Text = 0
                txtprostock.Text = 0
                txtnum.Text = 0
                lsvbuy.Items.Clear()

            End If
        End If


    End Sub


End Class