Both of these SQL statements return results:

Code:
SELECT person_id, person_first, person_last
FROM people, people_expertise, expertise2people
WHERE expertise2people.person_lookup_id = people.person_id 
AND expertise2people.expertise_lookup_id = people_expertise.expertise_id 
AND expertise_lookup_id = 5
GROUP BY person_id
Code:
SELECT person_id, person_first, person_last
FROM people, people_expertise, expertise2people
WHERE expertise2people.person_lookup_id = people.person_id 
AND expertise2people.expertise_lookup_id = people_expertise.expertise_id 
AND expertise_lookup_id = 6
GROUP BY person_id
How come when I combine the expertise I get no results?

Code:
SELECT person_id, person_first, person_last
FROM people, people_expertise, expertise2people
WHERE expertise2people.person_lookup_id = people.person_id 
AND expertise2people.expertise_lookup_id = people_expertise.expertise_id 
AND expertise_lookup_id = 5 AND  expertise_lookup_id = 6
GROUP BY person_id
I know there are people in the DB with both expertise 5 and 6. I want only those people. Thanks!