I have a database table which link locations together; a location can be in a location, which can be inside another location.
location (<id>, ....)
location_parent (<location_id>, <parent_id>)
$sql = "SELECT id FROM se_locations_services WHERE parent_locationid IN
( SELECT location_id FROM se_locations_parent WHERE parent_id = '$locationid' )";
There's a good-looking article over at mysql.com outlining various ways of managing hierarchical data. I think it provides a full solution to your question, and shows various less simple, but faster approaches (e.g. Nested Sets).