Question

I’d like to set up the following database scenario:

CREATE TABLE IF NOT EXISTS `points` (
  `po_id` INT UNSIGNED NOT NULL AUTO_INCREMENT,
  `po_north` INT,
  `po_east` INT,
  PRIMARY KEY (`po_id`),
) ENGINE=InnoDB;

CREATE TABLE IF NOT EXISTS `lines`(
  `li_id` INT UNSIGNED NOT NULL AUTO_INCREMENT,
  `li_from` INT NOT NULL,
  `li_to` INT NOT NULL,
  PRIMARY KEY (`li_id`),
  FOREIGN KEY (`li_from`) REFERENCES points(`po_id`),
  FOREIGN KEY (`li_to`) REFERENCES points(`po_id`),
) ENGINE=InnoDB;

Now I want to set up a third table, that sores some metadata like who created or altered a point or a line:

CREATE TABLE IF NOT EXISTS `metadata` (
  `me_type` ENUM('point','line') NOT NULL,
  `me_type_id` INT UNSIGNED NOT NULL,
  `me_permissions` VARCHAR(255) NOT NULL,
  `me_created_by` INT UNSIGNED NOT NULL,
  `me_created_on` DATETIME NOT NULL,
  `me_last_modified_by` INT UNSIGNED NOT NULL,
  `me_last_modified_by` DATETIME NOT NULL,
) ENGINE=InnoDB;

My first approach was to set an ENUM with two types (points and lines). But the problem is still, that I cannot properly reference a foreign key to one of the tables. Is there any recommended solution for such problem in MySQL?

BTW:
The fields for me_created_by and me_last_modified_by shall reference to a table storing some user data.

Answer 1

Your tables points and lines should contain a foreign key to metadata – not the other way around. Doing so will save you from defining any more complicated table setups. Using this approach, a single metadata-entry could be re-used several times for many different points or lines. This isn’t even MySQL specific but a general, normalized database structure.