how the textbox fetch the total price column in database using vb.net

here is my code and i want when i add purchase the textbox6 will change into total of the suprice

Code:

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Imports System.Data
Imports System.Data.OleDb

Public Class Form2
    Dim myconn As OleDbConnection = New OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0;Data Source=C:\Users\Marian Erana Ando\Desktop\NullBux\db.mdb")
    Dim cmd As OleDbCommand
    Dim dr As OleDbDataReader
    Dim str As String

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

        GroupBox2.Visible = False
    End Sub

    Public Function GetData() As DataView
        Dim SelectQry = "SELECT * FROM item "
        Dim SampleSource As New DataSet
        Dim TableView As DataView
        Try
            Dim SampleCommand As New OleDbCommand()
            Dim SampleDataAdapter = New OleDbDataAdapter()
            SampleCommand.CommandText = SelectQry
            SampleCommand.Connection = myconn
            SampleDataAdapter.SelectCommand = SampleCommand
            SampleDataAdapter.Fill(SampleSource)
            TableView = SampleSource.Tables(0).DefaultView
        Catch ex As Exception
            Throw ex
        End Try
        Return TableView
    End Function

    Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
        str = "INSERT INTO purchase(prno,dte,total,pay,stat) VALUES ('" & TextBox1.Text & "','" & DateTimePicker1.Text & "','" & TextBox6.Text & "','" & TextBox6.Text & "','" & TextBox8.Text & "')"
        cmd = New OleDbCommand(str, myconn)
        myconn.Open()
        cmd.ExecuteReader()
        myconn.Close()
    End Sub

    Private Sub Button1_Click_1(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

    End Sub

    Private Sub DataGridView1_CellClick(ByVal sender As Object, ByVal e As System.Windows.Forms.DataGridViewCellEventArgs) Handles DataGridView1.CellClick
       
    End Sub

    Private Sub Button3_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button3.Click
        Me.Close()
    End Sub

    Private Sub Button4_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button4.Click
        GroupBox2.Visible = True
        DataGridView2.DataSource = GetData()
    End Sub

    Private Sub Button6_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button6.Click
        GroupBox2.Visible = False
    End Sub

    Private Sub DataGridView2_CellClick(ByVal sender As Object, ByVal e As System.Windows.Forms.DataGridViewCellEventArgs) Handles DataGridView2.CellClick

        TextBox2.Text = DataGridView2.CurrentRow.Cells(0).Value.ToString
        TextBox3.Text = DataGridView2.CurrentRow.Cells(1).Value.ToString
    End Sub

    Private Sub DataGridView2_CellContentClick(ByVal sender As System.Object, ByVal e As System.Windows.Forms.DataGridViewCellEventArgs) Handles DataGridView2.CellContentClick
       
    End Sub

    Private Sub TextBox2_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles TextBox2.TextChanged
        TextBox4.Text = ""
        TextBox5.Text = ""
    End Sub

    Private Sub Button5_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button5.Click

        str = "INSERT INTO ptable(prno,itemno,suprice,qty) VALUES ('" & TextBox1.Text & "','" & TextBox2.Text & "','" & TextBox5.Text & "','" & TextBox4.Text & "')"
        cmd = New OleDbCommand(str, myconn)
        myconn.Open()
        cmd.ExecuteReader()
        myconn.Close()
        DataGridView1.DataSource = pur()
    End Sub
    Public Function pur() As DataView
        Dim SelectQry = "SELECT itemno,suprice,qty FROM ptable where prno ='" & TextBox1.Text & "' "
        Dim SampleSource As New DataSet
        Dim TableView As DataView
        Try
            Dim SampleCommand As New OleDbCommand()
            Dim SampleDataAdapter = New OleDbDataAdapter()
            SampleCommand.CommandText = SelectQry
            SampleCommand.Connection = myconn
            SampleDataAdapter.SelectCommand = SampleCommand
            SampleDataAdapter.Fill(SampleSource)
            TableView = SampleSource.Tables(0).DefaultView
        Catch ex As Exception
            Throw ex
        End Try
        Return TableView
    End Function
    Private Sub TextBox6_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles TextBox6.TextChanged

    End Sub

    Private Sub TextBox5_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles TextBox5.TextChanged
        Dim totalSum As Integer

        For i As Integer = 0 To DbDataSet.Tables(0).Rows.Count - 1
            totalSum += DbDataSet.Tables(0).Rows(i).Item("suprice")
        Next

        TextBox6.Text = totalSum.ToString()
    End Sub
End Class

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please help