Update inventory when sale is made vb.net - mysql

Question

I have a SalesForm whereby a user can add their sales. In my inventory table I have total tires, quantity in stock, and quantity sold. I know I have duplicate fields (total tires and quantity in stock), but only the quantity in stock will be used for updating and total tires will be used for referring to check how much they have sold out and which ones are being sold out fast.
What I am trying to do is after I click on save on SalesForm to add new sales, the inventory table should also be updated. The quantity in stock and quantity sold should add how much sold from the sales form and get saved to inventory.
But the calculation isn't working. I see the same information after the update.
This is the code for update:
Public Sub updatestock()
MysqlConn = New MySqlConnection
MysqlConn.ConnectionString = "server=localhost;userid=root;password=root;database=golden_star"
Dim a As Integer
' Dim total, onstock, quantity As String
Dim READER As MySqlDataReader
Try
MysqlConn.Open()
Dim Query As String
' Dim Sda As MySqlDataAdapter
a = Val(txtStock.Text) - Val(ComboBox3.Text)
'total = txtStock.Text
'quantity = ComboBox3.Text
'onstock = total - quantity
Query = "update inventory set quantity_onstock = '" & a & "' where brand = '" & ComboName.Text & "' and size = '" & ComboSize.Text & "' "
Command = New MySqlCommand(Query, MysqlConn)
MessageBox.Show("Stock Updated Successfully")
READER = Command.ExecuteReader
Catch ex As MySqlException
MessageBox.Show(ex.Message)
Finally
MysqlConn.Dispose()
End Try
End Sub
Screenshot frontend:
Screenshot database as it saves the same number it doesn't do the calculation:
Then I pasted the method name updatestock() on button click after clicking on save, but it's not working. Can anybody correct me with the update statement?

Answer 1

Here is a MUCH better way to structure the query code:
Public Sub UpdateStock(newQty As Integer, brand As String, size As String)
Dim SQL As String = "UPDATE Inventory SET quantity_onstock = #Qty WHERE brand = #Brand AND size = #Size;"
Using cn As New MySqlConnection("server=localhost;userid=root;password=****;database=golden_star"), _
cmd As New MySqlCommand(SQL, cn)
cmd.Parameters.AddWithValue("#Qty", newQty)
cmd.Parameters.AddWithValue("#Brand", brand)
cmd.Parameters.AddWithValue("#Size", size)
cn.Open()
cmd.ExecuteNonQuery()
End Using
End Sub
Note the Using block, the query parameters, the function arguments instead of form controls, and the lack of error handling or messaging. The method is concerned with running the query and ONLY running the query. You should adopt these changes for ALL your queries. Also note the use of ExecuteNonQuery() instead of ExecuteReader().
What this means is you also need to change the code that calls the function. It will need to handle the messages and errors, and provide the parameter arguments.
Whether this fixes your issue is hard to say.
As a further step from here, you can then start moving these SQL methods into their own module, away from any forms code, to create meaningful separation of concerns.