SQL SELECT query for multiple tables

Question

I have these tables:

USERS:
user_ID
username
password
email
data_registered

POSTS:
post_ID
user_ID
post_title
post_content
post_data

CATEGORY:
cat_ID
post_ID
cat_title

Now I used this situation:

in USERS table I have:

(1, 'admin, '5hj63jhn120011', 'admin@gmail.com', '15.02.2012')

in POSTS table:

(1, 1, 'Hello', 'Content goes here', '15.02.2012')

in CATEGORY table:

(1, 1, 'web developing'), 
(2, 1, 'software programming'), 
(3, 1, 'web design')

So I have only 1 user, 1 post, and 3 categories. How to select them in one statement? I used this statement:

SELECT 
  p.post_title, 
  p.post_content, 
  u.username, 
  c.cat_title 
FROM 
  posts p 
  LEFT JOIN users u 
     ON p.user_ID = u.user_ID 
  LEFT JOIN category c 
     ON c.post_ID = p.post_ID

but this select 3 rows with 3 different categories. I'm still learning to make this JOIN queries properly so I would appreciate any help.

And after that, for example, I will have situation to select data from COMMENTS table as well, so this is going to be complicated. Just to mention this is comment table:

COMMENTS:
comment_ID
user_ID
post_ID
c_username
c_email
c_content

It's gonna be tough to select this comments with user , post and category altogether. So one more question would be can I split this select queries and is this a good practice generaly (when developing with PHP) ?

Answer 1

Try:

SELECT p.post_title, 
       p.post_content, 
       u.username, 
       group_concat(c.cat_title) categories
FROM posts p 
LEFT JOIN users u ON p.user_ID = u.user_ID 
LEFT JOIN category c ON c.post_ID = p.post_ID
group by p.post_id

- if you want to see all the categories for a single post in the same row.

Answer 2

Use something like that:

SELECT 
  p.post_title, 
  p.post_content, 
  u.username, 
  GROUP_CONCAT(c.cat_title + ',') cat_titles
FROM posts p 
LEFT JOIN users u 
  ON p.user_ID = u.user_ID 
LEFT JOIN category c 
  ON c.post_ID = p.post_ID
GROUP BY 
  p.post_title, 
  p.post_content, 
  u.username

I'm not sure in proper usage of GROUP_CONCAT, but looks like if will work

Answer 3

I think it should be like this instead:

POSTS:
post_ID
user_ID
cat_ID
post_title
post_content
post_data

CATEGORY:
cat_ID
cat_title

As categories don't need posts to relate to but each post will need a category to relate to.

Update:

If many-to-many is the intention then it should be like this:

POSTS:
post_ID
user_ID
post_title
post_content
post_data

CATEGORY:
cat_ID
cat_title

POST_CATEGORY:
post_ID
cat_ID

Answer 4

without have tried it, I think the solution is:

SELECT p.post_title, p.post_content, u.username, c.cat_title 
FROM users u LEFT JOIN posts p ON u.user_ID = p.user_ID
LEFT JOIN category c ON p.post_ID = c.post_ID

if you change the order of the ID columns, the meaning of LEFT JOIN changes. So you get three result rows....