Foreign ID in SQL tables to link one table to another - mysql

Question

This is my SQL code which links users to items based on tutorials:
CREATE TABLE IF NOT EXISTS `users` (
`user_id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`user_name` varchar(64) COLLATE utf8_unicode_ci NOT NULL,
`user_password_hash` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
`user_email` varchar(254) COLLATE utf8_unicode_ci NOT NULL,
`user_access_level` tinyint(3) unsigned NOT NULL DEFAULT '0',
`user_active` tinyint(1) NOT NULL DEFAULT '0',
`user_activation_hash` varchar(40) COLLATE utf8_unicode_ci DEFAULT NULL,
`user_password_reset_hash` char(40) COLLATE utf8_unicode_ci DEFAULT NULL,
`user_password_reset_timestamp` bigint(20) DEFAULT NULL,
`user_failed_logins` tinyint(1) NOT NULL DEFAULT '0',
`user_last_failed_login` int(10) DEFAULT NULL,
`user_registration_datetime` datetime NOT NULL DEFAULT '0000-00-00 00:00:00',
`user_registration_ip` varchar(39) COLLATE utf8_unicode_ci NOT NULL DEFAULT '0.0.0.0',
PRIMARY KEY (`user_id`),
UNIQUE KEY `user_name` (`user_name`),
UNIQUE KEY `user_email` (`user_email`)
) ENGINE=MyISAM AUTO_INCREMENT=1 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
CREATE TABLE IF NOT EXISTS `item` (
`item_id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`item_title` varchar(64) COLLATE utf8_unicode_ci NOT NULL,
`item_location` varchar(64) COLLATE utf8_unicode_ci NOT NULL,
`item_description` varchar(64) COLLATE utf8_unicode_ci NOT NULL,
`item_datetime` datetime NOT NULL DEFAULT '0000-00-00 00:00:00',
`item_status` int(1) unsigned NOT NULL,
PRIMARY KEY (`item_id`)
) ENGINE=MyISAM AUTO_INCREMENT=1 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
http://sqlfiddle.com/#!9/fd011
I'm getting a bit confused about how to link the items to the user. It seems like I need something called a foreign key on the items, a bit like this in my item table:
FOREIGN KEY (user_id) REFERENCES user(ID)
I can't seem to get it to compile and query successfully. Can anyone please show me the right way to associate the items with the user.

Answer 1

You need to add the user_id column to the items table along with the constraint:
CREATE TABLE IF NOT EXISTS `users` (
`user_id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`user_name` varchar(64) COLLATE utf8_unicode_ci NOT NULL,
`user_password_hash` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
`user_email` varchar(254) COLLATE utf8_unicode_ci NOT NULL,
`user_access_level` tinyint(3) unsigned NOT NULL DEFAULT '0',
`user_active` tinyint(1) NOT NULL DEFAULT '0',
`user_activation_hash` varchar(40) COLLATE utf8_unicode_ci DEFAULT NULL,
`user_password_reset_hash` char(40) COLLATE utf8_unicode_ci DEFAULT NULL,
`user_password_reset_timestamp` bigint(20) DEFAULT NULL,
`user_failed_logins` tinyint(1) NOT NULL DEFAULT '0',
`user_last_failed_login` int(10) DEFAULT NULL,
`user_registration_datetime` datetime NOT NULL DEFAULT '0000-00-00 00:00:00',
`user_registration_ip` varchar(39) COLLATE utf8_unicode_ci NOT NULL DEFAULT '0.0.0.0',
PRIMARY KEY (`user_id`),
UNIQUE KEY `user_name` (`user_name`),
UNIQUE KEY `user_email` (`user_email`)
) ENGINE=MyISAM AUTO_INCREMENT=1 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
CREATE TABLE IF NOT EXISTS `item` (
`item_id` int(11) unsigned NOT NULL AUTO_INCREMENT,
user_id int unsigned,
`item_title` varchar(64) COLLATE utf8_unicode_ci NOT NULL,
`item_location` varchar(64) COLLATE utf8_unicode_ci NOT NULL,
`item_description` varchar(64) COLLATE utf8_unicode_ci NOT NULL,
`item_datetime` datetime NOT NULL DEFAULT '0000-00-00 00:00:00',
`item_status` int(1) unsigned NOT NULL,
PRIMARY KEY (`item_id`),
FOREIGN KEY (user_id) REFERENCES users(user_id)
) ENGINE=MyISAM AUTO_INCREMENT=1 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
The SQL Fiddle is here.
I should point out a few other things:
Pay attention to the database engine you are using. MyISAM doesn't actually enforce the relationship.
Having weird dates as the default value is probably less useful than just using NULL.
I'm not sure if there is a value to having explicit collations for every character definition, unless your database is going to be supporting a wide variety of collations.
Don't use single quotes for numeric constants. So, if a value is declared as a tinyint, set the default ot 0 not '0' (this doesn't affect performance in a CREATE TABLE statement; it is just misleading).

Answer 2

#GordonLindoff's solution is one method, but that assumes that each item belongs to exactly one user, and an item cannot be referenced by multiple users. If you have a many-to-many relationship, where a user can have multiple items and an item can be referenced by multiple users, then you need a third table that links them together:
CREATE TABLE IF NOT EXISTS `user_item` (
`user_id` int(11) unsigned NOT NULL,
`item_id` int(11) unsigned NOT NULL,
PRIMARY KEY (`user_item`,`item_id`),
FOREIGN KEY (user_id) REFERENCES users(user_id),
FOREIGN KEY (item_id) REFERENCES items(item_id)
)ENGINE=Innodb DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
The Foreign Key constraints enforce that for every row in user_item that the user_id exists in users, and the item_id exists in items. And as was mentioned in a previous comment, that you will need Innodb to have the foreign key constraints enforced.