title: CREATE UNIQUE TOKEN(NUMERIC)
body: <p>I have a database where I want to generate unique tokens of six numeric characters. This tokens should be generated each time an invoice number is inserted in the <code>invoice_no</code> column. This would mean that every time I do an insert and the column <code>invoice_no</code> is not <code>null</code>, the token is generated of the numerals. I want the token to be to generate as many tokens as possible without a token being repeated at any instance. Thanks</p>