Use Two Conjection Table in a Single Query? - php

Question

I am creating a blog post system in which I am using following table structure :-
blog_category
category_id, category_name, enabled, created_date
blog_post
post_id,title,article, author_id,date_published
post_category
category_id, post_id
blog_tag
tag_id, tag_name
post_tag
id,tag_id,post_id
post_related
id, post_id, post_related_id
blog_comment
comment_id, post_id, is_reply_to_id,comment,user_id
My questions ARE:
How to get this type of output in a single query
title,author_name,article,total_comments,post_tags
How to insert records in post_related table to show the related post when select a particular post.
I trying like this but it show unknown column bp.post_id
CREATE PROCEDURE blog_get_posts_in_category(
IN inCategoryId INT, IN inShortPostDescriptionLength INT,
IN inPostsPerPage INT, IN inStartItem INT)
BEGIN
PREPARE statement FROM
"SELECT bp.post_id, bp.title,(select count(*) from blog_comment where post_id=bp.post_id) as total_comments,
IF(LENGTH(bp.article) <= ?,
bp.article,
CONCAT(LEFT(bp.article, ?),
'...')) AS article,
DATE_FORMAT(bp.date_published,'%d %M %Y at %h:%i:%s %p') as date_published, bp.banner_image,ba.display_name
FROM blog_post bp
INNER JOIN blog_post_to_category bpc
ON bpc.post_id = bp.post_id
INNER JOIN blog_author ba
ON ba.id = bp.author_id
WHERE bpc.category_id = ? and enabled=1
ORDER BY bp.post_id DESC
LIMIT ?, ?";
SET #p1 = inShortPostDescriptionLength;
SET #p2 = inShortPostDescriptionLength;
SET #p3 = inCategoryId;
SET #p4 = inStartItem;
SET #p5 = inPostsPerPage;
EXECUTE statement USING #p1, #p2, #p3, #p4, #p5;
END$$

Answer 1

Q1:
This is not really clear from the question, so lets suppose that
- blog_author table looks like
author_id, author_name, ...
- total_comments is the number of all the comments belonging to a blog post.
- post_tags could be one string containing all the post_tags related to a blog post (e.g. as a csv).
Then, a query could be:
SELECT
bp.post_id as pid, bp.title, a.author_name, bp.article, bc.total_comments, pt.post_tags
FROM blog_post bp
LEFT JOIN blog_author a ON bp.author_id=a.id
LEFT JOIN (
SELECT post_id, COUNT(*) as total_comments
FROM blog_comment
GROUP BY post_id) bc
ON bc.post_id=bp.post_id
LEFT JOIN (
SELECT post_id, GROUP_CONCAT(DISTINCT tag_id SEPARATOR ',') AS post_tags
FROM post_tag
GROUP BY post_id) pt
ON pt.post_id=bp.post_id;
Q2:
Here, the query seems to be OK. I even tried it myself and I got no error message about the bp.post_id. Just one idea that the subquery with the total_comments could be avoided using the join (with the blog_comment table) that I show in my previous query.
But this procedure/query does something different than the question supposed to be about. (It does not even have any reference to the blog_related table.)