If you have a lot of tips, you could create a unique indexed tip number
column. Select the highest tip number using:
select tip_number from tips order by tip_number desc limit 1;
Then generate a random number and select using that tip_number.
Of course, you would have to allow for the possibility of missing tip
numbers, by repeating the random number generation/read sequence until you
find something. Since the tip_number isn't the PK of the table, you can
regenerate the tip numbers to eliminate holes from deletions any time you
like. Just reset the sequence to 1 and update all rows with the
nextval(tipnumber_seq).
Sounds like a lot of work to me though...