I was thinking of formatting it like this

TYYYYMMDDNNNNNNNNNNX (1 character + 19 digits)

Where

• T is type
• YYYY is year
• MM is month
• DD is day
• N is sequencial number
• X is check digit

The problem is, how do I generate the sequencial number? since my primary key is not an auto increment integer value, if it was i would use that, but its not.

EDIT can I have the sequencial number resets itself after 1 day (24hours).

• P201012080000000001X <-- first transaction of 2010/12/08
• P2010120810000000002X <--- second transaction of 2010/12/08
• P201012090000000001X <--- First transaction of 2010/12/09

(X is the check digit)