prevent an update from occurring under certain conditions

[Carol Cheung <cacheung(at)consumercontact(dot)com>]

Hi,
I'm not sure where to look for information about how to accomplish this:

TABLES
state:
 id | state_name | active
----+------------+--------
  1 | Washington | t
  2 | Illinois   | t

county:
 id | county_name | active | state_id
----+-------------+--------+----------
  1 | King        | t      |        1
  2 | Jefferson   | t      |        1

I want to prevent an update to set a state's active to false if there 
are any counties in that state that are active (county.active = true and 
county.state_id = state.id)
Is this possible?

Thanks in advance for your help.