Joined SQL query MAX aggregate with condition - sql

Question

I am having trouble with a SQL query. Here is a representation of my schema on SQL Fiddle:
http://sqlfiddle.com/#!15/14c8e/1
The issue is that I want to return rows of data from the Invitations table and join them with a sum of both the 'sent' event_type and 'viewed' event_type from the associated events, as well as the latest created_at date.
I can get all the data and counts working, but am having issue with the last_sent_on. Is there a way I can use a condition in a MAX aggregate function?
e.g.
MAX(
SELECT events.created_at
WHERE event_type='sent'
)
If not, how would I write the proper subselect?
I am currently using Postgresql.
Thank you.

Answer 1

You can use a case statement inside of max just as you've done with sum. The query below will select the maximum created_at for event_type='sent'
SELECT
i.id,
i.name,
i.email,
max(case when e.event_type='sent' then e.created_at end) AS last_sent_on,
sum(case when e.event_type='sent' then 1 else 0 end) AS sent_count,
sum(case when e.event_type='viewed' then 1 else 0 end) AS view_count
FROM
invitations i
LEFT OUTER JOIN
events e
ON e.eventable_id = i.id
WHERE e.eventable_type='Invitation'
GROUP BY i.id, i.name, i.email
SQLFiddle

Answer 2

Try using a subquery to build the max value for sent.
SELECT
i.id,
i.name,
i.email,
sent.last_sent,
sum(case when e.event_type='sent' then 1 else 0 end) AS sent_count,
sum(case when e.event_type='viewed' then 1 else 0 end) AS view_count
FROM
invitations i
LEFT OUTER JOIN
events e
ON e.eventable_id = i.id
LEFT JOIN ( SELECT eventable_id uid, MAX(created_at) AS last_sent
FROM events
WHERE event_type = 'sent'
GROUP BY eventable_id ) AS sent
ON sent.uid = i.id
WHERE e.eventable_type='Invitation'
GROUP BY i.id, i.name, i.email, sent.last_sent