Help with a microblog (twitter clone) database structure?

I'm working on an exercise that wants me to create a small twitter clone, with users, tweets and following system. Well, i came up with the following database structure:

CREATE TABLE tweets (
  tweet_id INT NOT NULL AUTO_INCREMENT,
  tweet VARCHAR(140) NOT NULL,
  PRIMARY KEY (tweet_id)
) ENGINE=INNODB;


CREATE TABLE users (
  user_id INT NOT NULL AUTO_INCREMENT,
  user VARCHAR(255) NOT NULL,
  password VARCHAR(40) NOT NULL,
  email VARCHAR(255) NOT NULL,
  PRIMARY KEY (user_id)
) ENGINE=INNODB;

CREATE TABLE user_tweets (
  id INT NOT NULL AUTO_INCREMENT,
  id_user INT NOT NULL,
  id_tweet INT NOT NULL,
  PRIMARY KEY(id),
  FOREIGN KEY (id_tweet)
    REFERENCES tweets(tweeth_id)
      ON UPDATE NO ACTION ON DELETE NO ACTION,
  FOREIGN KE开发者_如何学JAVAY (id_user)
    REFERENCES users(user_id)) ENGINE=INNODB;

CREATE TABLE followers (
  id_user INT NOT NULL REFERENCES users (user_id),
  id_following INT NOT NULL REFERENCES users (user_id),
  PRIMARY KEY (id_user, id_following)
) ENGINE=INNODB;

Is it valid? Am i missing something? Also:

• How do i select the tweets from a user?
• How do i select the followers from a user?
• How do i select the people a user is following?

I'm getting a little lost with the foreign key concept. :(

Building on the answer by @Karel, I'd use slightly different tables:

CREATE TABLE tweets (
  tweet_id INT NOT NULL AUTO_INCREMENT,
  user_id INT NOT NULL
  tweet VARCHAR(140) NOT NULL,
  PRIMARY KEY (tweet_id),
  FOREIGN KEY user_id(user_id) REFERENCES users(user_id) 
  ON UPDATE CASCADE ON DELETE CASCADE
) ENGINE=INNODB;


CREATE TABLE users (
  user_id INT NOT NULL AUTO_INCREMENT,
  user VARCHAR(255) NOT NULL,
  /*password VARCHAR(40) NOT NULL,*/<<--- NEVER STORE A PASSWORD IN THE CLEAR!
  passhash VARCHAR(40) NOT NULL,
  email VARCHAR(255) NOT NULL,
  PRIMARY KEY (user_id)
) ENGINE=INNODB;


CREATE TABLE followers (
  id_user INT NOT NULL REFERENCES users (user_id),
  id_following INT NOT NULL REFERENCES users (user_id),
  PRIMARY KEY (id_user, id_following)
) ENGINE=INNODB;

How do i select the tweets from a user?

SELECT * FROM tweets WHERE user_id = 458

How do i select the followers from a user?

SELECT * FROM users 
INNER JOIN followers ON (users.users_id = followers.id_user) 
WHERE followers.id_following = 458

How do i select the people a user is following?

SELECT * FROM users
INNER JOIN followers ON (followers.id_following = users.user_id)
WHERE followers.id_user = 458

Use a SHA2 hash to compare the password.
And don't forget to add a salt to the hashing to prevent rainbow attacks.

SELECT user_id 
FROM users 
WHERE users.user = 'OralB' 
      AND users.passhash = SHA2(CONCAT(users.user,'secretToothbrush'),512)

SHA1 is no longer secure, so I'd advice using SHA2 with a 512bit hash length.

Links
MySQL tutorial: http://www.tizag.com/mysqlTutorial/
Foreign keys: http://dev.mysql.com/doc/refman/5.5/en/innodb-foreign-key-constraints.html
SHA2: http://dev.mysql.com/doc/refman/5.5/en/encryption-functions.html#function_sha2
Concat: http://dev.mysql.com/doc/refman/5.0/en/string-functions.html#function_concat
Why salt: What is "salt" when relating to MYSQL sha1?

You don't need a table 'user_tweet' because a tweet can only belong to 1 user, so it would be more logical to have a 'user_id' in you tweet-table

a crosstable like user_tweet is only useful when there is a many-to-many relationship (for example a teacher teaches multiple classgroups, classgroups get classes from multiple teachers)

if you do this you can select the tweets from a user by this sql-statement:

SELECT Tweet
FROM Tweets t (your table name)
WHERE t.User_ID == UserID_Whose_Tweets_you_want

you should now be able to get the followers with a similar query ;D

Hope you can do anything with this!