Distinct select where row in one of multiple tables - sql

Question

I'm building a feed for a project, and an object can appear in the feed depending from a couple of places.
I have a table of posts, which have a group and a visibility. Some require membership to view, others are public.
posts
id
group_id
visibility
1
1
public
2
1
member
3
2
public
4
2
member
Membership is recorded in a members table, with a start and end date for the membership:
members
id
user_id
group_id
period_start
period_end
1
1
1
2020-01-01
2020-02-01
2
2
2
2020-01-01
2020-01-01
At the moment, my query returns just the posts from the groups the user is a member of:
SELECT posts.*
FROM members
LEFT JOIN posts
ON
posts.group_id = members.group_id AND
posts.visibility IN ('member', 'public') AND
posts.deleted_at IS NULL
WHERE
members.user_id = XYZ AND
members.period_start < now() AND
members.period_end > now();
I want to add an the ability for a user to add a group to their favorites without becoming a member, which will show just the public posts in their feed.
favorites
id
user_id
group_id
1
1
2
How can I change the above query so it selects posts from the users membership groups, and also the public posts from their favorited groups?
It needs to be distinct, so if the user is both a member and has favourited the post doesn't appear twice.
I'm expecting the result to look like the below for user_id 1:
id
group_id
visibility
1
1
public
2
1
member
3
2
public

Answer 1

I think you just want union all:
SELECT p.*
FROM members m JOIN
posts p
ON p.group_id = m.group_id AND
p.visibility IN ('member', 'public') AND
p.deleted_at IS NULL
WHERE m.user_id = XYZ AND
m.period_start < now() AND
m.period_end > now()
UNION ALL
SELECT p.*
FROM favorites f JOIN
posts p
ON p.group_id = f.group_id AND
p.visibility IN ('public') AND
p.deleted_at IS NULL
WHERE f.user_id = XYZ ;
Note the joins are inner joins, not outer joins.
I'm not sure if the favorites need the filter on active members or not; this does not include it.
You can also express this with two LEFT JOINs and some filtering:
SELECT p.*
FROM posts p LEFT JOIN
favorites f
ON f.group_id = p.group_id AND
p.visibility IN ('public') LEFT JOIN
members m LEFT JOIN
ON p.group_id = m.group_id AND
p.visibility IN ('member', 'public') AND
m.period_start < now() AND
m.period_end > now()
WHERE p.deleted_at IS NULL AND
XYZ IN (m.user_id, f.user_id)