Question

I have an objects table and votes table. I am currently grouping them and sorting by most recent vote:

SELECT objects.id, objects.name, MAX(votes.created_at) AS mv 
       FROM "objects"
       INNER JOIN "votes" ON "votes"."object_id" = "objects"."id" 
       GROUP BY objects.id, objects.name 
       ORDER BY mv DESC

However, this only returns the objects that have votes. I searched on SO and found answers regarding how to include results that don’t exist, but I’m looking to include both those that exist and those that do not.

Is there a way to tack onto the end of the above all of the objects that don’t have votes as well?

Answer 1

With a LEFT JOIN, NULL values are filled in where no matching rows are found – as you found out yourself:

super frustrating because it works perfectly, except all of the
entries without votes are at the beginning!

Now, NULL is normally sorted last. But reversed with DESC it comes first. In Postgres you can use NULLS LAST to explicitly sort NULL to the end (since version 8.3):

SELECT o.id, o.name, max(v.created_at) AS lastest_vote
FROM   objects o
LEFT   JOIN votes v ON v.object_id = o.id 
GROUP  BY o.id, o.name
ORDER  BY lastest_vote DESC NULLS LAST, o.name; -- name only as tiebreaker

If objects.id is the primary key you can simplify the GROUP BY (in Postgres 9.1 or later):

GROUP  BY o.id -- primary key covers all columns of this table