Left Join most recent record, if one exists - mysql

Question

I have a table of people, and I want to pull the most recent record of event, but there may not be one.
So my problem is that even though I'm left joining to make sure I'm getting a record for each person, I'm only bringing back people that have events because I'm pulling the Max of a date as a criteria.
SELECT *
FROM tbl_people
LEFT JOIN tbl_events ON
tbl_people.People_UID = tbl_events.People_UID
WHERE tbl_people.Active = 1
AND (
SELECT MAX(Event_Date) FROM tbl_events
)
Table People would have the following
People_UID
Table Events would have the following
Event_UID, People_UID, Event_Name, Event_Date
Like I said, what I'd like is an output like this:
Jen, Had a Baby, 7/10/2015
Shirley
Susan
Megan, Had a Baby, 8/5/2014
etc.
I hope that makes sense.

Answer 1

select * from
( SELECT *, row_number() over (partition by tbl_people.name order by tbl_events.Event_Date desc) as rn
FROM tbl_people
LEFT JOIN tbl_events
ON tbl_people.People_UID = tbl_events.People_UID
WHERE tbl_people.Active = 1 ) t
where t.rn = 1

Answer 2

It depends on the database.
If you're using MySQL, you could simply put the correlated subquery into the on clause of the outer join like this:
select *
from tbl_people p
left join tbl_events_latest e
on p.people_uid = e.people_uid
and e.event_date =
(select max(x.event_date) from tbl_events x where x.people_uid = e.people_uid)
where p.active = 1
In other databases, that might not be so. For example, in Oracle, you'd get an error. You could alternatively run:
with tbl_events_latest as
(select *
from tbl_events e
where event_date = ( select max(x.event_date)
from tbl_events x
where x.people_uid = e.people_uid ) )
select *
from tbl_people p
left join tbl_events_latest e
on p.people_uid = e.people_uid
where p.active = 1

Answer 3

One way to do this is to use a correlated subquery in the join that checks that there doesn't exists any event for the same person with a later date:
SELECT *
FROM tbl_people p
LEFT JOIN tbl_events e
ON p.People_UID = e.People_UID
AND NOT EXISTS (
SELECT 1 FROM tbl_Events
WHERE e.People_UID = People_UID
AND Event_Date > e.Event_Date
)
WHERE p.Active = 1
It might not be the most efficient solution though; maybe limiting the left joined set first is better. (Or come to think of it the exists should be better).
Given a sample data set like:
People_UID Event_UID People_UID Event_Name Event_Date
Jen 1 Jen Had a Baby 2015-07-10
Jen 3 Jen Bought a horse 2013-07-10
Shirley NULL NULL NULL NULL
Susan NULL NULL NULL NULL
Megan 2 Megan Had a Baby 2014-08-05
Megan 4 Megan Had another Baby 2015-08-05
This would be the result:
People_UID Event_UID People_UID Event_Name Event_Date
Jen 1 Jen Had a Baby 2015-07-10
Shirley NULL NULL NULL NULL
Susan NULL NULL NULL NULL
Megan 4 Megan Had another Baby 2015-08-05
Using joins it could look like this:
SELECT *
FROM tbl_people p
LEFT JOIN (
SELECT e.*
FROM tbl_events e
JOIN (
SELECT PEOPLE_UID, MAX(EVENT_DATE) MDATE
FROM tbl_Events GROUP BY People_UID
) A ON A.MDATE = E.event_date AND e.People_UID = A.People_UID
) B ON p.People_UID = b.People_UID
WHERE p.Active = 1

Answer 4

In case of Sql Server you can use OUTER APPLY:
SELECT *
FROM tbl_people p
OUTER APPLY(SELECT TOP 1 Event_Date
FROM tbl_events e
WHERE p.People_UID = e.People_UID
ORDER BY Event_Date DESC)oa
WHERE p.Active = 1
Or if you want to select just date then:
SELECT *
FROM tbl_people p
OUTER APPLY(SELECT MAX(Event_Date) AS Event_Date
FROM tbl_events e
WHERE p.People_UID = e.People_UID)oa
WHERE p.Active = 1