Question

If I have three tables:

posts
------
postid  date_posted title   content


views
------
viewid  post_id user_id


users
------
userid  username

1. What is the correct way of listing all posts that have not been viewed by user_id? Is this query correct?
   
   SELECT * FROM posts LEFT JOIN views ON postid=post_id WHERE user_id=123 AND postid=NULL;

2. How about listing all posts and their “view status” by user_id?

3. Can you also recommend a great (user-friendly, non-geeky) resource to improve my understanding of LEFT JOIN, CROSS JOIN, and (normal joins? select * from posts, views)?

Thank you!

Answer 1

1.

SELECT *
FROM posts
WHERE postid NOT IN (
    SELECT post_id 
    FROM views
    WHERE user_id = 123
)

OR

SELECT *
FROM posts
LEFT JOIN views ON views.post_id = posts.postid AND views.user_id = 123
WHERE viewid IS NOT NULL