如何从VB.net中的两个不同表中减去两列？

Question

我刚刚开始使用VB.net并且知道我还是新手，我很难从两个不同的表中减去名为Quantity and Stocks的列（1.Sales和2.AddItem）我想要的每当我按下名为＆＃34;购买＆＃34; ，它会减去数量＆＃34;数量＆＃34;在＆＃34; Stocks＆＃34;的数量。可能吗？感谢。

    Dim connectString As String
    Dim conn As SqlConnection
    Dim adapter As New SqlDataAdapter
    Dim sql As String
    connectString = "Data Source=LENOVO\MYSQL;Integrated Security=True;database = CellSalesandInventory"
    conn = New SqlConnection(connectString)
    sql = "insert into Sales (NameofUser,ProductCode,Brand,Model,NameofCustomer,Age,Sex,Address,Date,Quantity,Warranty,Price) values ('" & TextBox10.Text & "','" & TextBox1.Text & "',
   '" & TextBox6.Text & "','" & TextBox11.Text & "','" & TextBox2.Text & "','" & TextBox3.Text & "','" & ComboBox1.Text & "','" & TextBox4.Text & "',
   '" & TextBox5.Text & "','" & TextBox7.Text & "','" & TextBox8.Text & "','" & TextBox9.Text & "') "
    Try
        conn.Open()
        adapter.InsertCommand = New SqlCommand(sql, conn)
        adapter.InsertCommand.ExecuteNonQuery()
        MessageBox.Show("You just purchased a product!Thank You!")
    Catch ex As Exception
        MsgBox(ex.ToString)
End Sub