How to implement SQL Foreign Keys in Parent Tables

Question

First Question here:

Currently I am working on a project that requires the use of 3 different Entities. Those Entities are: Users, Venues and Facilities.

Each Entity has its table:

Facility Table:

CREATE TABLE `facility` (
`facility_id` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
`venue_id` INT(10) UNSIGNED NOT NULL,
`name` VARCHAR(50) NOT NULL COLLATE 'utf8_unicode_ci',
`surface` VARCHAR(50) NOT NULL COLLATE 'utf8_unicode_ci',
`max_timeblock` INT(10) UNSIGNED NOT NULL,
`min_timeblock` INT(10) UNSIGNED NOT NULL,
`availability` INT(10) UNSIGNED NOT NULL,
PRIMARY KEY (`facility_id`),
UNIQUE INDEX `facility_id` (`facility_id`),
INDEX `facility-venue_id_key` (`venue_id`),
CONSTRAINT `facility-venue_id_key` FOREIGN KEY (`venue_id`) REFERENCES `venue` (`venue_id`) ON UPDATE CASCADE ON DELETE CASCADE)

Venue Table:

CREATE TABLE `venue` (
    `venue_id` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
    `name` VARCHAR(50) NOT NULL COLLATE 'utf8_unicode_ci',
    `description` VARCHAR(50) NOT NULL COLLATE 'utf8_unicode_ci',
    `email` VARCHAR(50) NOT NULL COLLATE 'utf8_unicode_ci',
    `phone` VARCHAR(50) NOT NULL COLLATE 'utf8_unicode_ci',
    `opening_time` TIME NOT NULL,
    `closing_time` TIME NOT NULL,
    `booking_conditions` VARCHAR(50) NOT NULL COLLATE 'utf8_unicode_ci',
    `booking_method` INT(10) UNSIGNED NOT NULL,
    `status` INT(10) UNSIGNED NULL DEFAULT NULL,
    PRIMARY KEY (`venue_id`),
    UNIQUE INDEX `venue_id` (`venue_id`)
)

User Table:

CREATE TABLE `user` (
    `user_id` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT COMMENT 'This field will hold the Unique ID for each user.',
    `username` VARCHAR(50) NOT NULL COMMENT 'This field will be classed as a display name for the users so when searching they arent looking for the ID.' COLLATE 'utf8_unicode_ci',
    `email` VARCHAR(50) NOT NULL COLLATE 'utf8_unicode_ci',
    `password` VARCHAR(128) NOT NULL COLLATE 'utf8_unicode_ci',
    `first_name` VARCHAR(50) NOT NULL COLLATE 'utf8_unicode_ci',
    `last_name` VARCHAR(50) NOT NULL COLLATE 'utf8_unicode_ci',
    `facebook_id` VARCHAR(50) NULL DEFAULT NULL COLLATE 'utf8_unicode_ci',
    `user_type_id` INT(10) NULL DEFAULT NULL,
    `postcode` INT(10) NULL DEFAULT NULL,
    `status` INT(10) NULL DEFAULT NULL,
    `date_created` TIMESTAMP NOT NULL DEFAULT '0000-00-00 00:00:00',
    `date_modified` TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
    PRIMARY KEY (`user_id`),
    UNIQUE INDEX `username` (`username`),
    UNIQUE INDEX `email` (`email`),
    UNIQUE INDEX `user_id` (`user_id`)
)

As such, Im trying to implement A Media Table.

An example of this table is below:

CREATE TABLE `media` (
    `media_id` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
    `album_id` INT(10) UNSIGNED NULL DEFAULT '0',
    `alt_name` VARCHAR(50) NOT NULL DEFAULT '0' COLLATE 'utf8_unicode_ci',
    `mine_type` VARCHAR(50) NOT NULL DEFAULT '0' COLLATE 'utf8_unicode_ci',
    `url_link` VARCHAR(50) NOT NULL DEFAULT '0' COLLATE 'utf8_unicode_ci',
    PRIMARY KEY (`media_id`),
    INDEX `media-album_id_key` (`album_id`),
    CONSTRAINT `media-album_id_key` FOREIGN KEY (`album_id`) REFERENCES `album` (`album_id`) ON UPDATE CASCADE ON DELETE CASCADE
)

What I want is to Allow a User, Facility to have 1 image associated with it and a Venue to have a Album associated with it.

As such Im not sure how to implement the foreign keys in this regards because if I put a column of media into the Users table and set a foreign key to the Media_ID if I delete the Media it deletes the User as the cascade affect does this.

What I want is that if I delete the User/Facility/Venue it will delete all images associated with it.

Any assistance would be appreciated.

Regards,

Robert

Answer 1

You cannot do what you want to do directly without using triggers, so I've listed that here as your first option, along with a second option that you really should avoid but will give you a clue as to how to investigate this pattern problem further.

• Keep the foreign key from each facility/venue/user to the media table, but use a ON DELETE SET NULL rather than ON DELETE CASCADE referential action, and then implement the deletion of related media in your application code (or in triggers)
• Look up "polymorphic associations" and why they're a bad idea in relational databases, and if you're still convinced you want to do that, use one of the available techniques to construct the inverted relationship you're thinking of doing.