如何在SQL数据库中扣除数量？

Question

enter image description here I'm trying to get the difference of Quantity and ItemBought in my Database and update the Quantity column based on what is the difference between the two.

I tried different SQL code for this one, but I always got the same error

Try getConnection()

        sql = "SELECT (Quantity - ItemBought) AS [Quantity] FROM products WHERE ProductCode = @ProductCode;"

        cmd = New MySqlCommand
        With cmd
            .Connection = conn
            .CommandText = sql
            .Parameters.Clear()

            .Parameters.AddWithValue("@ProductCode", formPOS.ProductCodeTB.Text)
            .Parameters.AddWithValue("@ItemBought", formInventory.quantityTB.Text)
            result = cmd.ExecuteNonQuery()
            If result = 0 Then
                MsgBox("Error in updating the selected product!")
            Else
                MsgBox("Successfully updated the selected product!")
            End If"

Answer 1

UPDATE the table and with the modified value and refill the gridview. It will directly change the table value.

Or

If you are looking for temporary change of data means follow the below step:

"SELECT Quantity AS [Quantity] FROM products WHERE ProductCode = @ProductCode;"

Then subtract the ItemBought then display on the gridview.

Answer 2

You are trying to exececutenonquery() so I believe you trying to update your products table.

 sql = "UPDATE products set Quantity = (Quantity - @ItemBought) WHERE ProductCode = @ProductCode;"

        cmd = New MySqlCommand
        With cmd
            .Connection = conn
            .CommandText = sql
            .Parameters.Clear()

            .Parameters.AddWithValue("@ProductCode", formPOS.ProductCodeTB.Text)
            .Parameters.AddWithValue("@ItemBought", formInventory.quantityTB.Text)
            result = cmd.ExecuteNonQuery()
            If result = 0 Then
                MsgBox("Error in updating the selected product!")
            Else
                MsgBox("Successfully updated the selected product!")
            End If"

Answer 3

Just got the right answer, I solved it by setting the right TextBox and right Form of the column ProductCode

Try
        getConnection()

        sql = "UPDATE products SET Quantity = (Quantity - @ItemBought) WHERE ProductCode = @ProductCode;"

        cmd = New MySqlCommand
        With cmd
            .Connection = conn
            .CommandText = sql
            .Parameters.Clear()

            .Parameters.AddWithValue("@ProductCode", formPOS.ProductCodeTB.Text)
            .Parameters.AddWithValue("@ItemBought", formQuantity.quantityTB.Text)
            result = cmd.ExecuteNonQuery()
            If result = 0 Then
                MsgBox("Error in updating the selected product!")
            Else
                MsgBox("Successfully updated the selected product!")
            End If
        End With