Question

Pretty simple question, just cannot wrap my head around it.
Example:
3 tables. Owner <-> OwnerAnimal <-> Animal

Animal can have multiple owners, and owner can have multiple animals.
Now, given a specific owner, find other owners that have animals in common with the given owner.

What I think is that we need to do join on the same table multiple times, like this:

select distinct  
o2.Owner_Id,   
o2.Name  
from Owner o  
left join OwnerAnimal  oa  
on o.Owner_Id = oa.Owner_Id   
left join OwnerAnimal oa2   
on oa.Animal_id = oa2.Animal_Id   
left join Owner o2   
on. oa2.Owner_Id = o2.Animal_Id   
Where o.Owner_Id = 100 and o2.Owner_Id <> 100 --To exclude current owner from the list

But I’m not sure if this is a right approach.

Answer 1

If you want any overlap of animals, the following is the way that I think of it:

select distinct ao.owner
from AnimalOwners ao
where ao.animal in (select animal from AnimalOwners ao1 and ao1.owner = 100) and
      ao.owner <> 100

You can rewrite this as a join, but the in seems to make more sense.

If you want all animals to be the same, then you need to do a join.

with a as (select distinct animal from AnimalOwners where ao.owner = 100)
select ao.owner
from AnimalOwners ao left outer join
     a
     on ao.animal = a.animal
where ao.owner <> 100
group by ao.owner
having count(disinct ao.animal) = (select count(*) from a) and
       count(*) = count(a.animal)

The idea is to do a set comparison using the having clause. The first subclause guarantees that the number of animals for the second owner is the same as the number for 100. The second guarantees that there is no animal owned by the second owner that is not owned by the original owner. The left outer join keeps all animals.

The use of the distinct keyword is for the situations where an animal could appear twice for an owner. It is unclear whether this is allowed.

If you want owners that own the same animals as the original owner, but could own others, then change the left outer join to a join in the previous query.