I've seen a lot of google results for this problem (a weighted random select from mysql) that claim to give you a result, but do not actually work. This is the solution I use:
select * from random_table order by -LOG(1.0-RAND())/weight limit 1
Obviously you need a field named weight, I use 5 (rarest), 10 (uncommon), 20 (common). Since I use this for procedural generation, and not at "runtime" where the player would be hitting the query a lot, I'm not overly concerned about performance.
Relevant links:
http://www.kahunaburger.com/2008/10/13/selecting-random-weighted-records-from-mysql/http://use.perl.org/~bart/journal/33630
