Question

I am designing a site like pinterest ( for study ) and I want to know what query I have to do at the homepage to show the user’s stream.

I have created these 6 tables:

users
boards
pictures
boards_pictures ( many-to-many table )
followers
comments

And in home, I do this query to get all followers’ pictures.

SELECT users.username, pictures.link, comments.comment, boards.title
FROM boards_pictures, pictures, followers, users, comments, boards 
WHERE ( boards_pictures.id_user = followers.id_following ) 
  AND ( boards_pictures.id_picture = pictures.id ) 
  AND ( followers.id_user = $_session['userid'] ) 
  AND ( users.id = followers.id_following )
  AND ( comments.picture_id = pictures.id )
  AND ( boards.id = boards_pictures.boards_id )

Is there a way to avoid this complex query ( a JOIN with 6 tables ) ?

Answer 1

Here’re the DDL statements deduced from your query:

CREATE TABLE users (id integer, username varchar(30));
CREATE TABLE boards (id integer, title varchar(30));
CREATE TABLE pictures (id integer, link varchar(90));
CREATE TABLE boards_pictures (
  id_user integer,
  id_picture integer,
  boards_id integer);
CREATE TABLE followers (id_user integer, id_following integer);
CREATE TABLE comments (picture_id integer, comment varchar(350));

You’re mixing column naming styles here, compare users.id, followers.id_user, comments.picture_id and board_pictures.id_picture (last 2 are quite misleading). The more tables you’ll have, the more attention you’ll have to pay on how column is named. It’s better to stick to a single common pattern, picture_id or user_id being most appropriate in my view.

Your query is not complicated, except you’re using implicit join notation. It is not the recommended way, as there’s a chance to miss some predicates and end up with Cartesian product of 2 (or more) tables.

Your query can be rewritten like this:

SELECT u.username, p.link, c.comment, b.title
  FROM boards_pictures bp
  JOIN pictures p ON p.id = bp.id_picture
  JOIN followers f ON bp.id_user = f.id_following
  JOIN users u ON u.id = f.id_following
  JOIN comments c ON c.picture_id = p.id
  JOIN boards b ON b.id = bp.boards_id
WHERE f.id_user = $_session['userid'];

As you see now, query is really simple and has only one predicate.
I’ve created a testbed without data on SQL Fiddle.

In my view your structure is quite good. There’s no need to change neither table design nor this query.