multiple datagridview data update to database mysql - vb.net

Question

I have created a shopping system with two different interfaces. The admin interface is used to add/update/delete items to the database and the customer interface to purchase products. I have a problem updating the quantity in the admin interface after the user bought the product. The quantity in the admin interface is updated till the value goes negative.

Try
    For Each row As DataGridViewRow In formPOS.ProductAddToCartDGV.Rows
        dbconnection()
        sql = "UPDATE products_database SET Product_Quantity = Product_Quantity - @ProductQuantity WHERE Product_Code = @ProductCode;"
        cmd = New MySqlCommand
        With cmd
            .Connection = conn
            .CommandText = sql
            .Parameters.Clear()
            .Parameters.AddWithValue("@ProductCode", row.Cells(0).Value)
            .Parameters.AddWithValue("@ProductQuantity", row.Cells(3).Value)
            result = .ExecuteNonQuery()

            If result = 0 Then
                MsgBox("Error in updating stocks")
            Else
                MsgBox("Successfully update stocks")
            End If
        End With
    Next
Catch ex As Exception
    MsgBox(ex.Message)
Finally
    conn.Close()
End Try

Answer 1

Do you want to ensure that the value entered by customer is less than or equal to the remaining total? If so, you can add a condition Product_Quantity >= CAST(@ProductQuantity AS int) in WHERE clause.

For i As Integer = 0 To CustomerDGV.Rows.Count - 2
    Dim constr As String = "connection string"
    Dim sql As String = "UPDATE products_database SET Product_Quantity = Product_Quantity - @ProductQuantity WHERE Product_Code = CAST(@ProductCode AS int) AND Product_Quantity >= CAST(@ProductQuantity AS int)"

    Using conn As SqlConnection = New SqlConnection(constr)
        conn.Open()
        Dim cmd As SqlCommand = New SqlCommand()
        cmd.Connection = conn
        cmd.CommandType = CommandType.Text
        cmd.CommandText = sql
        cmd.Parameters.AddWithValue("@ProductCode", CustomerDGV.Rows(i).Cells(0).Value)
        cmd.Parameters.AddWithValue("@ProductQuantity", CustomerDGV.Rows(i).Cells(1).Value)
        Dim rows As Integer = cmd.ExecuteNonQuery()

        If rows = 0 Then
            MessageBox.Show($"Make sure {CustomerDGV.Rows(i).Cells(0).Value} ProductQuantity <= Product_Quantity")
        End If
    End Using
Next

Note: this is a SQL Server demo.