This query searches the external table for people who have family search IDs and does a cross table query against the events and places table to find people matching that criteria who also have birth events recorded as "unknown" countries. Since virtually everyone who has a family search ID has a recorded birth place this should aid in cleaning up those entries in the events table.
I am adding a lot of family search IDs to the database but often am not taking the time to update the events table with the information from family search. This will make it much easier to identify those entries.