MySQL Limit By Second Table - mysql

Question

I've got a database with events and organizations that run those events. I am looking to create a query that shows any organization that has not created an event in 90 days or more.
So far, I have this query:
SELECT organizations.name, organizations.first_name, organizations.last_name,
organizations.email, events.created_at, events.start_date, events.end_date
FROM events
INNER JOIN organizations ON events.organizer_id = organizations.id
WHERE DATE_SUB(CURDATE(),INTERVAL 90 DAY) > events.created_at
GROUP BY events.organizer_id
ORDER BY events.created_at DESC
The problem is that this will just choose any event that is more than 90 days old, but not the latest event. How do I get the query to look at the event with the newest created_at and see if that is 90 days or older and include only that?

Answer 1

You can use aggregation. If you care about organizations, then you don't need event information in the select:
SELECT o.name, o.first_name, o.last_name, o.email
FROM events e INNER JOIN
organizations o
ON e.organizer_id = o.id
GROUP BY o.id
HAVING DATE_SUB(CURDATE(), INTERVAL 90 DAY) > MAX(e.created_at)
ORDER BY MAX(e.created_at) DESC;
This query will not select organizations that never have events. To do that, you need a left outer join. Here is one way:
SELECT o.name, o.first_name, o.last_name, o.email
FROM organizations o LEFT JOIN
events e
ON e.organizer_id = o.id AND
e.created_at >= DATE_SUB(CURDATE(), INTERVAL 90 DAY)
WHERE e.organizer_id is null
GROUP BY o.id
ORDER BY MAX(e.created_at) DESC;
Note that I also changed the query to use table aliases. These make the query easier to write and read.

Answer 2

Try this:
First create a query (call it "A" here) that selects all organiazations which created an event in the last 90 days.
SELECT organizer_id from events where DATE_SUB(CURDATE(),INTERVAL 90 DAY) <= events.created_at
Then write a query which lists all organizations which are not in the result set of query A:
SELECT * from organizations where id not in (A)
Which makes it to:
SELECT * from organizations where id not in (SELECT organizer_id from events where DATE_SUB(CURDATE(),INTERVAL 90 DAY) <= events.created_at)
This works because SQL is "orthogonal", i.e., you can embed queries into other queries.