Generating Auto ID in Database - vb.net

Question

i have a table that captures the details of Products. I want to be able to generate an Auto ID that contains special characters
I have been able to write code that generates the auto ID and also add a new one. But the issue arises when it gets to the 10th and 11th record.
It seems to be seeing the 9th record as the MAX in the database.
this makes it to throw error that there will be duplicate.
For example, record 9 generates CAT009, record 10 generates CAT0010 but instead to generate record 11 as CAT0011, it seems to keep generating CAT0010 because of the MAX function i used in the SELECT statement.
Sub auto()
Try
Dim cn As New OleDbConnection("Provider=Microsoft.ACE.OLEDB.12.0;Data Source=" & Application.StartupPath & "\inventory1.accdb")
cn.Open()
Dim dr As OleDbDataReader
Dim cmddr As New OleDbCommand
cmddr.CommandText = "SELECT MAX(category_id) as max_id FROM tblcategory "
cmddr.Connection = cn
dr = cmddr.ExecuteReader
dr.Read()
If IsDBNull(dr("max_id")) Then
autonumber = "CAT00" & 1
Else
Dim str As String
str = dr.Item("max_id").ToString()
Dim P As Double
Dim N As Double
N = Len(str)
P = N - 5
autonumber = "CAT00" & Convert.ToInt32(str.Substring(5, P))+ 1
End If
cn.Close()
Catch Ex As Exception
MsgBox(Ex.Message)
Console.WriteLine(Ex.Message)
End Try
End Sub

Answer 1

The overall concept is a duplicate of other questions, the key concern being proper formatting of the number with correct zero padding. There are various ways to do this in VB.Net, but the Int32.ToString(String) accepts a format string that can handle zero-padding. It's also unnecessary to calculate the string length, since Substring() has an overload to get all remaining characters to the right.
If IsDBNull(dr("max_id")) Then
autonumber = "CAT001"
Else
Dim str As String
str = dr.Item("max_id").ToString()
autonumber = "CAT" & (Convert.ToInt32(str.Substring(3)) + 1).ToString("000");
End If
If the first two zeros should be static (i.e. always "CAT00"), then it is critical to still format the changing portion with proper padding. Otherwise the text strings will not sort properly and so the numeric order will be ineffective. In that case, you should use something like
If IsDBNull(dr("max_id")) Then
autonumber = "CAT00001" '* NOT CAT001
Else
Dim str As String
str = dr.Item("max_id").ToString()
autonumber = "CAT00" & (Convert.ToInt32(str.Substring(5)) + 1).ToString("000");
End If
Of course in either case, be aware that the number of integer digits limits the range of valid numbers. For example, 3 digits ("000") has a maximum of 999.
A better approach is to use pure integer AutoNumbers in the database: 132 -> "CAT132". Then just format the number for display purposes, and when necessary parse user input text to extract the integer portion: "CAT0089" -> 89. This approach highly simplifies the backend code and leaves the AutoNumber functionality to the database itself... no need to have custom code to generate the numbers.