i can't think how to do it with your table schema in a single query, but i can at least offer a (admittedly kind of ugly) solution:

you could add a last_action column to the lb_objs table which you update everytime you insert or update the row. you could probably also define a trigger to automatically keep the lb_trans table up to date (avoiding having to do 2 update/inserts each time).

anders pearson // digital samurai
personal      // http://www.columbia.edu/~anders/  
weblog       // http://thraxil.dhs.org/