Get user Posts and/or Comments in a sharing blog using SQL

问题

This (My)SQL problem will make me crazy if not solved this week-end!

A simple shared blog (means many authors contribute). Let's consider that two tables on my database:

Posts:

id
author_id
title
content

Comments:

id
author_id
post_id
content

Goal: I want to display all the activities of a contributor (=> fixed author_id). What I mean by activity is:

If the user created the post and commented: Display the post title and the user comment
If the user created the post but doesn't comment: Display only post title
If the user didn't create the post, but commented: Display the post title and the user comment

I tried this SQL Script:

SELECT p.id AS post_id, p.author_id AS post_author_id, c.author_id AS comment_author_id, title, c.content

FROM Posts p JOIN Comments c ON p.id = c.post_id

WHERE p.author_id = $userId

OR c.author_id = $userId

Looks fine, but it doesn't give me the row where the user created the post, but doesn't comment.

Any idea? Thanx in advance.

回答1:

To simulate FULL OUTER JOIN in MySQL, you need to UNION the results of Posts outer joined to Comments, with those of Comments outer joined to Posts - like so:

SELECT p.id AS post_id, 
       p.author_id AS post_author_id, 
       c.author_id AS comment_author_id,
       p.title, 
       c.content
FROM Posts p 
LEFT JOIN Comments c ON p.id = c.post_id AND c.author_id = $userId
WHERE p.author_id = $userId
UNION ALL
SELECT p.id AS post_id, 
       p.author_id AS post_author_id, 
       c.author_id AS comment_author_id,
       p.title, 
       c.content
FROM Posts p 
RIGHT JOIN Comments c ON p.id = c.post_id
WHERE c.author_id = $userId

来源：https://stackoverflow.com/questions/9530760/get-user-posts-and-or-comments-in-a-sharing-blog-using-sql

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