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;