SQL查询-不同的结果

People [*ID*, Name] 
Pet [*PetID*, OwnerID, Species, Name]

ID    | No. of Species
----------------------
1473  | 2

count(distinct ...)

select  People.ID
,       count(distinct Species)
from    People
join    Pet
on      Pet.OwnerID = People.ID
group by
        People.ID

select people.name, count(distinct pet.species)
from people, pet
where people.id = pet.ownerid
group by people.name

Select ID,[No. of Species] from People 
inner join 
  ( select Count(Species) as [No. of Species],OwnerID from Pet  
   group by OwnerID) d 
on Id = d.OwnerID