непосредственный быстрый nhibernate?

Я думаю, что функция доступа LAST решит вашу проблему.

Private Function GetLastID() As String
    Dim LastID As String = ""
    Try
        'The Using blocks ensure that your database objects are closed and disposed even if there is an error
        Using cn As New OleDbConnection("Provider=Microsoft.ACE.OLEDB.12.0;Data Source=" & Application.StartupPath & "\inventory1.accdb")
            'Pass the query and connection directly in to the constructor of the command
            'The Access LAST function returns the field specified for the last record.
            Using cmd As New OleDbCommand("SELECT Last(category_id) FROM tblcategory;", cn)
                'Open the connection at the last minute.
                cn.Open()
                'Since you only need a single value you can use .ExecuteScalar
                LastID = cmd.ExecuteScalar().ToString
            End Using 'close and dispose the command
        End Using 'close and dispose the connection
    Catch Ex As Exception
        MessageBox.Show(Ex.Message)
    End Try
    Return LastID
End Function

Private Function GetNewID(LastID As String) As String
    Dim NewID As String
    'The substring starts at a specified character position And continues to the end of the string.
    NewID = "CAT00" & Convert.ToInt32(LastID.Substring(5)) + 1
    Return NewID
End Function

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    'Assumes there is at least one record in the table
    Dim LastID As String = GetLastID()
    Dim NewID As String = GetNewID(LastID)
End Sub