if mysql, if you have a table with a primary id and another table with ratings, and I wanted to get all the items that have a usability rating of 8 or more, I would do the following...

Tables like...
create table items (
  itemid int(11) default 0 not null,
  other_data text,
  primary key (itemid);
); 

create table ratings (
  itemid int(11) default 0 not null,
  ratingtype varchar(32) default '' not null,
  rating int(11) default 0 not null,
  primary key (itemid,ratingtype),
  key (itemid,ratingtype,rating)
};

SELECT * FROM items AS I, ratings AS R WHERE R.itemid = I.itemid 
AND R.ratingtype = 'usability' AND R.rating >= 8;
[download]

and that should work...

I just used int for rating for simplicity, you'd probably use a decimal...
Or are there multiple ratings for on item in the same ratingtype? like an entry for each person who rarted it? If that is the case I would suggest just storing the average if you can, or having a table of averages and a table of all entries. but maybe I misunderstand.
                - Ant