MySQL：联接三个表

Question

I'm new to this so please forgive the basic question. I'm just having a hard time conceptualizing how this would work.

Say I have three tables in my database with these relevant columns:

POSTS - post_id, post_title, post_date
CATEGORIES - post_id, cat_id
CATEGORY NAMES - cat_id, cat_name

Now, I'm trying to display a list of all posts sorted by the post_date. In this list, I'd like to list the category.

I've gotten as far as this:

SELECT * 
FROM posts
LEFT JOIN categories ON posts.post_id = categories.post_id
ORDER BY post_date ASC

This sort of works but has two problems:

1. Since posts can have multiple categories (ie multiple entries in categories per post_id ), I'm getting duplicates. I'd like to get rid of the duplicates but acknowledge if a post is in multiple categories.
2. I don't know how to go the step further and get the cat_name other than doing another query. This seems a bit wasteful though. Is there a way to do it in one query?

Again, sorry for the basic nature of this. I've been reading tutorials for a long time but I can't seem to wrap my head around this. Any direction is appreciated.

Answer 1

One row per post, eh? How about we give you the category names comma-separated?

     SELECT p.post_title, p.post_date, 
            GROUP_CONCAT(c.cat_name SEPARATOR ', ') AS categories
       FROM posts AS p
  LEFT JOIN categories AS pc ON p.post_id = pc.post_id
  LEFT JOIN category_names AS c ON pc.cat_id = c.cat_id 
   GROUP BY p.post_title, p.post_date
   ORDER BY p.post_date ASC, p.post_title

The two consecutive joins pick up the category name, and the GROUP_CONCAT aggregates multiple category names into a comma-separated list. For GROUP_CONCAT to work we also need GROUP BY .

Answer 2

use this format:

SELECT t1.name
              ,t1.age
              ,GROUP_CONCAT(t2.job_name SEPARATOR',')typeofjob
        FROM t_one AS t1
        JOIN t_three AS t3 ON t3.user_id = t1.user_id
        JOIN t_two AS t2 ON t2.job_id = t3.job_id
        GROUP BY t1.user_id;

---

output: