An example

We’ll create two tables, a users and account table. The account table holds account names (e.g. Guest, Admin, Super Admin) and the users table contains one foreign key that relates to the account type.

The structure looks like this;

And here’s some data that we’ll work with…

Now to perform a simple join of these two tables, we could use…

This simple query will return all records but using this as a base we can further customise our query to make it more useful. If we wanted to find out all users who have Admin accounts we’d use this statement.

SELECT * FROM users u LEFT JOIN accounts a ON u.account_type = a.id WHERE a.name = 'Admin'

// or we could use the ID from the accounts table

SELECT * FROM users u LEFT JOIN accounts a ON u.account_type = a.id WHERE a.id = 2

A note on usage

This is MySQL join in its most simplest terms. You can join multiple tables providing you database design allows it. So with that cleared up, if you are not already using joins in your queries, start using them and cut down on the number of transactions you make…it’ll also make your life a lot easier retrieving data.

Connect via SSH and login to mysql…

# mysql -u root -p

// now grant access for the root account
mysql > GRANT USAGE ON *.* to root@'%' IDENTIFIED BY '[password]';
mysql > FLUSH PRIVILEGES;

Change [password] to whatever the password is for the root account and include the single quotes (‘). The % sign means it will accept connections from this account on all IP addresses. You could if you wanted, choose a specific IP here.

You’ll now be able to login remotely.

A typical example

Let’s say you have a table called users and a user_meta table that holds multiple rows relating to the user table (use the following code in a test database).

CREATE TABLE `users` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `username` varchar(30) DEFAULT NULL,
  `passwd` varchar(40) DEFAULT NULL,
  `email` varchar(120) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=latin1;

CREATE TABLE `user_meta` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `meta_name` varchar(255) DEFAULT NULL,
  `meta_value` text,
  `user_id` int(10) unsigned NOT NULL,
  PRIMARY KEY (`id`),
  KEY `FK_user_meta_users` (`user_id`),
  CONSTRAINT `FK_user_meta_users` FOREIGN KEY (`user_id`) REFERENCES `users` (`id`) ON DELETE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=latin1 ROW_FORMAT=DYNAMIC;

Notice the database engine in this case is InnoDB. MyISAM does not support foreign keys. Without worrying too much about the syntax, the user_meta table declares that the column ‘user_id’ is foreign and references the primary key on the user table (id).

Now that they foreign keys are referenced what can we do with them? In this case, I’ve declared ON DELETE CASCADE. This means that if we delete a user from the user table, all user_meta records for that user will be deleted automatically. If we said on UPDATE CASCADE, should the users primary key get updated, then the user_meta table will automatically update the user_id column as necessary.

There are other options too…

RESTRICT and NO ACTION. These two are similar in that nothing will get changed upon the parent table changing. SET NULL will set the foreign key table column values to null upon delete/update. SET DEFAULT can also come in handy if you have set default values to columns.

Try them out, if you havent’ used them before, I will guarantee you will find uses for them in the future.

Download sample database dump: Foreign Key Tutorial SQL Dump (88)