Question

Not quite sure how to ask or define this, but can’t figure it out.

I have three tables like this:

persons             person_id, first_name, last_name
hobbies             hobby_id, name
persons_hobbies     person_id, hobby_id

I need to make two lists. Persons that have both hobby A and B, and persons that have hobby A but not B. How can I write these two queries? Can’t figure out how to do this with joining and all…

Say hobby A has id=3 and hobby B has id=7.

Answer 1

Something like this should work:

-- Persons that have both hobby A and B
select p.first_name,p.last_name
from persons p
inner join persons_hobbies ph1 on ph1.person_id = p.person_id and ph1.hobby_id = 3
inner join persons_hobbies ph2 on ph2.person_id = p.person_id and ph2.hobby_id = 7;

-- Persons that have hobby A but not B 
select p.first_name,p.last_name
from persons p
inner join persons_hobbies ph1 on ph1.person_id = p.person_id and ph1.hobby_id = 3
left outer join persons_hobbies ph2 on ph2.person_id = p.person_id and ph2.hobby_id = 7
where ph2.person_id is null;