How to write this SQL query to get approved posts - mysql

Question

I have 3 tables Users, posts and post_approval
Now I have page where all these posts will appear from posts table but if the post visibility is set to private then the query will check in post_approval table to see if a post status is approvad for a particular user.
So the Output for User ID 2 will be all 3 posts but User 1 & 3 can see only post id 1 and 3.
Is this the right query?
SELECT * FROM posts WHERE visiblity='public' UNION (SELECT posts.* FROM POSTS RIGHT JOIN post_approval ON post_approval.post=posts.post_id WHERE post_approval.status='approved' AND post_approval.user='2')
USER ID will be enter dynamically in the above query

Answer 1

Simple answer
Try this SQL statement.
SELECT * FROM Posts as p, Post_approval as pa
WHERE p.post_id = pa.post OR p.visibility = 'public' AND user_id = 2
You select from Posts and Post approval, and get only the posts which match with the the post id of the user. Furthermore you get all post with visibility public.
Long answer (union)
That's the longer solution. Using a union to combine the private post and public posts for user_id = 2.
SELECT
*
FROM
(
SELECT
u.user_id,
u.username,
p.post_id,
p.post_name,
p.visibility
FROM
Users as u,
Posts as p
WHERE
p.visibility = 'public'
union ALL
SELECT
u.user_id,
u.username,
p.post_id,
p.post_name,
p.visibility
FROM
Post_approval as a,
Users as u,
Posts as p
WHERE
a.user = u.user_id and
a.post = p.post_id and
p.visibility = 'private'
) as x
WHERE
x.user_id = 2
Another small tip:
If you have a foreign key in your tables, like Post_approval.user, name it like the primary key. So in this case Post_approval.user_id

Answer 2

I am going to answer your question before I offer an alternative. You have some spelling errors, an extra ( and you might need to be concerned with case-sensitivity, but otherwise it should work. Your SQL with some minor corrections:
SELECT * FROM Posts WHERE visibility='public'
UNION
SELECT Posts.* FROM Posts RIGHT JOIN Post_approval ON Post_approval.post=Posts.post_id
WHERE Post_approval.status='approved' AND Post_approval.user='2';
An alternative that does not use UNION:
select p.* from Posts p
left join Post_approval p_a on p.post_id = p_a.post and p_a.user = 2
where p.visibility = 'public' or p_a.status = 'approved';
See Db-Fiddle Demo
Another Way:
SELECT * FROM Posts
where visibility = 'Public' or post_id in (
select post from Post_approval where user = 2 and status = 'approved'
)