Auto-generate unique number for child tables
I have a one to many relationship (Site is the parent and ceramics, glass, metal are the children). I want to auto-generate a unique number, yet sequential, for each child record unrelated to the parent and across the entire database. So the first ceramic record I create is #1 then the next if it is glass would be #2, regardless of the site. How can I do this or is there a specific post that relates to this that I haven't found? Thanks.