How to select first row from a join that returns multple rows on the primary key

    A.PersonName, A.Email
        Select Person.PersonName, Email.Email
            ,ROW_NUMBER() OVER(PARTITION BY Person.ID ORDER BY Email.Email) AS RN
        From person 
        left join Email on Person.ID=Email.PersonId
        ) A

I would use an outer apply for this, I find it more readable.

Select Person.PersonName, coalesce(Email.Email,'No email found.') as Email
From person 
outer apply (
  select top(1) Email.Email 
  from Email 
  where Person.ID=Email.PersonId
  order by <whatever suits you>
) as Email;

As it does not matter which email shows up. I think that the following one is very direct.

Select Person.PersonName,  MIN(Email.Email)
From person 
left join email 
on Person.ID=Email.PersonId
group by Person.Id, Person.PersonName