Ticket Sales For Groups And Random Code ?

I need to provide a way to handle groups that buy tickets in bulk lots, say 20 at a time. Users can later register with a "ticket code". I need to tie these two together and provide a way for groups to buy more tickets at a later time.


There should be a code that can be generated for each ticket that a group buys. This ticket is then to be given to a user to register with. A rogue user should not find it easy to guess the code. So, I want to generate a code for each group as they register, that is a random 6 or 7 digit, alphanumeric string and then something similar for a ticket. See table definitions below.

So for a ticket code, we could have something like this:
A23486D_TR34, where A23486D would be the group code

comments/suggestions/ideas welcome

At present Ive built two tables, [groups] and [group_tickets]



[group]
id int(10) unsigned
name varchar(45)
phone varchar(10)
email varchar(45)
contact_namevarchar(100)
signup_datedatetime
last_updatedatetime
group_codevarchar(6)
addr1 varchar(45)
addr2 varchar(45)
city varchar(45)
state char(2)
zip varchar(10)

[group_ticket]
id int(10) unsigned
group_idint(10) unsigned
purchase_datedatetime
fill_datedatetime
ticket_notestext
ticket_code varchar(12)