Question

The title pretty much says it all. I have users posting content, and I would like to attach the user ID that is in the users table to the posts. Should I add a userid column in the posts table, should I join via the DB language, or should I use the scripting language to poll each table? If I should join, what kind of join should I do?

Sample content would look like this: “The teddy bears at my store are softer than the teddy goats, but the teddy goats are more lovable.” – Posted by James Teddyman at 3:36 PM.

Answer 1

A very often-used concept in situation like this is by having a users table and a post table, linking them together with a unique identifier. This identifier can be anything – a serialized id, a user name, mail address, etc – as long as it’s unique. Linking is done using a foreign key constraint. Exactly how this is achieved in MySQL I do not know, but in Postgres it’s done like this:

CREATE TABLE users (
  id serial PRIMARY KEY,
  name text
);

CREATE TABLE posts (
  content text,
  user_id integer REFERENCES users(id) NOT NULL
);

The tables are then merged using a join. This can be done in several ways, but here is a cross join after insertion of some values to play with:

@> INSERT INTO users (name) VALUES ('James');
@> INSERT INTO users (name) VALUES ('Jones');
@> INSERT INTO posts (content, user_id) VALUES ('Hello from James.', 1);
@> INSERT INTO posts (content, user_id) VALUES ('Greetings from Jones.', 2);
@> SELECT U.id AS user_id, U.name, P.content \
   FROM users U, posts P \
   WHERE U.id = P.user_id;

 user_id | name  |        content
---------+-------+-----------------------
       1 | James | Hello from James.
       2 | Jones | Greetings from Jones.

YMMV in MySQL, but I think the constructions above will work straight off.

(edit: Added INSERTs for clarification)