[c0ncrete]

yeah, it's called a subquery.

say for instance you have a table called known_pets that has a column, called npc_type_id. this would obviously store the id for npc types that you know are pets.

if you wanted to double the hitpoints for all pets, you'd use the following query:

UPDATE
    npc_types
SET
    hp = hp * 2
WHERE
    npc_types.id
IN
    (SELECT npc_type_id FROM known_pets)

if you wanted to do something to everything that isn't a pet, you'd just use NOT IN, instead of IN.

be aware that subqueries can only return a single column from a table. more info can be found here: http://dev.mysql.com/doc/refman/5.0/en/subqueries.html