Can I be selective on what rows I join on in MySQL - mysql

Question

Suppose I have two tables, people and emails. emails has a person_id, an address, and an is_primary:
people:
id
emails:
person_id
address
is_primary
To get all email addresses per person, I can do a simple join:
select * from people join emails on people.id = emails.person_id
What if I only want (at most) one row from the right table for each row in the left table? And, if a particular person has multiple emails and one is marked as is_primary, is there a way to prefer which row to use when joining?
So, if I have
people: emails:
------ -----------------------------------------
| id | | id | person_id | address | is_primary |
------ -----------------------------------------
| 1 | | 1 | 1 | a#b.c | true |
| 2 | | 2 | 1 | b#b.c | false |
| 3 | | 3 | 2 | c#b.c | true |
| 4 | | 4 | 4 | d#b.c | false |
------ -----------------------------------------
is there a way to get this result:
------------------------------------------------
| people.id | emails.id | address | is_primary |
------------------------------------------------
| 1 | 1 | a#b.c | true |
| 2 | 3 | c#b.c | true | // chosen over b#b.c because it's primary
| 3 | null | null | null | // no email for person 3
| 4 | 4 | d#b.c | false | // no primary email for person 4
------------------------------------------------

Answer 1

You got it a bit wrong, how left/right joins work.
This join
select * from people join emails on people.id = emails.person_id
will get you every column from both tables for all records that match your ON condition.
The left join
select * from people left join emails on people.id = emails.person_id
will give you every record from people, regardless if there's a corresponding record in emails or not. When there's not, the columns from the emails table will just be NULL.
If a person has multiple emails, multiple records will be in the result for this person. Beginners often wonder then, why the data has duplicated.
If you want to restrict the data to the rows where is_primary has the value 1, you can do so in the WHERE clause when you're doing an inner join (your first query, although you ommitted the inner keyword).
When you have a left/right join query, you have to put this filter in the ON clause. If you would put it in the WHERE clause, you would turn the left/right join into an inner join implicitly, because the WHERE clause would filter the NULL rows that I mentioned above. Or you could write the query like this:
select * from people left join emails on people.id = emails.person_id
where (emails.is_primary = 1 or emails.is_primary is null)
EDIT after clarification:
Paul Spiegel's answer is good, therefore my upvote, but I'm not sure if it performs well, since it has a dependent subquery. So I created this query. It may depend on your data though. Try both answers.
select
p.*,
coalesce(e1.address, e2.address) AS address
from people p
left join emails e1 on p.id = e1.person_id and e1.is_primary = 1
left join (
select person_id, address
from emails e
where id = (select min(id) from emails where emails.is_primary = 0 and emails.person_id = e.person_id)
) e2 on p.id = e2.person_id

Answer 2

Use a correlated subquery with LIMIT 1 in the ON clause of the LEFT JOIN:
select *
from people p
left join emails e
on e.person_id = p.id
and e.id = (
select e1.id
from emails e1
where e1.person_id = e.person_id
order by e1.is_primary desc, -- true first
e1.id -- If e1.is_primary is ambiguous
limit 1
)
order by p.id
sqlfiddle