VB Checking amount in SQL table and updating table in SQL - sql-server

Question

I need some help with my validation. What I'm supposed to do is check weather the number in the textbox is greater than or less than the quantity (stored in an SQL table called inventory) for that specific item (selected from the drop downlist). If the number in the textbox is greater than then quantity for that item then a message pops up saying that there isn't enough quantity for that item. If the number is less than the quantity in the SQL table for that item then it updates that quantity for that item. What my problem is, is that my validation isn't going to my second validation. It's like its stuck at the first if statement and I don't know how to fix it. I feel like its something very simple but I just can't put my finger on it. Here's my code:
Dim DA As New SqlDataAdapter("Select itemID, quantity From Inventory", con)
Dim DT As New DataTable
Try
If DT.Rows.Count > 0 Then DT.Rows.Clear()
DA.Fill(DT)
With DropDownList1
.DataSource = DT
.DataTextField = "itemID"
.DataValueField = "quantity"
.DataBind()
.Items.Insert(0, "Please Select a Customer")
End With
Catch ex As Exception
Response.Write(ex.Message)
End Try
End Sub
Protected Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Dim PurchaseQuantity, UP, SL As String
PurchaseQuantity = TextBox1.Text
Dim cmdUpdate As New SqlCommand("Update Inventory SET quantity = #p1 where price = #p2", con)
With cmdUpdate.Parameters
.Clear()
.AddWithValue("#p1", PurchaseQuantity)
.AddWithValue("#p2", DropDownList1.SelectedValue)
End With
Dim cmdSelect As New SqlCommand("Select quantity From Inventory where itemID = #p1", con)
With cmdSelect.Parameters
.Clear()
.AddWithValue("#p1", DropDownList1.SelectedValue)
End With
Try
If con.State = ConnectionState.Closed Then con.Open()
UP = cmdUpdate.ExecuteNonQuery
SL = cmdSelect.ExecuteScalar
Catch ex As Exception
Response.Write(ex.Message)
Finally
con.Close()
End Try
If PurchaseQuantity > SL Then
MsgBox("There is not enough quantity for this item", vbExclamation, "Error")
Else
If PurchaseQuantity < SL Then
MsgBox("Inventory is updated", vbExclamation, "Error")
UP -= PurchaseQuantity
End If
End If

Answer 1

So, you're seeing the error "There is not enough quantity for this item"?
I see a few problems:
Update Inventory SET quantity = #p1 where price = #p2
should be:
Update Inventory SET quantity = quantity - #p1 where itemID = #p2
Also, the order of the commands is backwards. The SELECT should happen before the UPDATE. (Otherwise, you're deducting the quantity from your inventory, THEN checking that you have enough to fill the order.)
Also, this whole operation should be transaction-bounded. You're open to a race condition, of sorts, with the current code. (If (2) simultaneous orders are placed, you could end up with a negative inventory quantity.) Or, at least, put a check constraint on the table (Inventory.quantity > 0) so it forces a SQL Exception in VB and halts the update.