Can't generate a StockID above 10 - sql

Question

This is the code that tries to grab the largest StockID from the database (Access database) , but my problem is that it generates StockID's up to "S10", after this it simply doesn't increment any further. This is the subroutine that generates the StockID:
Sub generate_Stock_ID()
Dim Stock_start As String = "S"
Dim Stock_Gen As String = "SELECT MAX(StockID) FROM tblStock WHERE StockID LIKE '" & Stock_start & "%%%' "
Dim da As OleDbDataAdapter = New OleDbDataAdapter(Stock_Gen, conn)
Dim ds As DataSet = New DataSet
da.Fill(ds, "StockID")
Dim dt As DataTable = ds.Tables("StockID")
Dim count As Integer = ds.Tables("StockID").Rows.Count
If ds.Tables("StockID").rows.count = 0 Then
StockID = "S1"
Else
StockID = ds.Tables("StockID").Rows(0).Item(0)
StockID = StockID.Substring(1, (StockID.Length - 1))
StockID = Stock_start & (StockID + 1)
End If
End Sub
Screenshot of my database
Note* there are multiple ID's for various other subroutines which all share the same incrementation issue, so if i fix this i fix the other ones too. So at the moment i think my problem lies in the syntax of my SQL statement, but im open to suggestions.
Thanks!

Answer 1

Don't treat an Integer as String. Otherwese MAX or ORDER BY will use lexicographical instead of numerical order which means that S11 is "lower" than S2.
So you should make this column an int-column and prepend S only where you display it. Then MAX(StockID) returns an Integer, you just have to cast it and add 1:
Using conn As New OleDbConnection("Connection-String")
Using cmd As New OleDbCommand(Stock_Gen, conn)
conn.Open()
Dim stockIDObj As Object = cmd.ExecuteScalar()
If stockIDObj IsNot Nothing Then
Dim maxStockId As Int32 = DirectCast(stockIDObj, Int32)
maxStockId += 1
' ...... '
End If
End Using
End Using
You should also change OPTION STRICT to ON. Then this would never compile since the same variable cannot be used for an Object, String and Integer which is very good since it prevents errors.
If you want to keep it as string you have to cast the substring always in the database which is less readable and less efficient. I also don't know how to do it in access.
If you want to change the type of column in an already populated table you should first add a new column with a similar name which is of type int. If all have S at the beginning you could first remove that, then you can update the new column with the casted int value. Finally you can delete the old column and rename the new to the old.

Answer 2

The root of this issue that StockID is a STRING and 'S1'>'S10' so for all StockId > 10 you get max = 'S1'.
As a fast fix try to change MAX(StockID) to:
SELECT 'S'+CAST(MAX(CAST(SUBSTRING(StockID,2,100) as int)) as varchar(100))
For ACCESS DB try to use:
SELECT "S" & cstr(MAX(CINT(MID(StockID,2,100))))