13.3 FOREIGN KEY Constraint Differences

The MySQL implementation of foreign key constraints differs from the SQL standard in the following key respects:

• If there are several rows in the parent table with the same referenced key value, InnoDB performs a foreign key check as if the other parent rows with the same key value do not exist. For example, if you define a RESTRICT type constraint, and there is a child row with several parent rows, InnoDB does not permit the deletion of any of the parent rows.

• If ON UPDATE CASCADE or ON UPDATE SET NULL recurses to update the same table it has previously updated during the same cascade, it acts like RESTRICT. This means that you cannot use self-referential ON UPDATE CASCADE or ON UPDATE SET NULL operations. This is to prevent infinite loops resulting from cascaded updates. A self-referential ON DELETE SET NULL, on the other hand, is possible, as is a self-referential ON DELETE CASCADE. Cascading operations may not be nested more than 15 levels deep.

• In an SQL statement that inserts, deletes, or updates many rows, foreign key constraints (like unique constraints) are checked row-by-row. When performing foreign key checks, InnoDB sets shared row-level locks on child or parent records that it must examine. MySQL checks foreign key constraints immediately; the check is not deferred to transaction commit. According to the SQL standard, the default behavior should be deferred checking. That is, constraints are only checked after the entire SQL statement has been processed. This means that it is not possible to delete a row that refers to itself using a foreign key.

• No storage engine, including InnoDB, recognizes or enforces the MATCH clause used in referential-integrity constraint definitions. Use of an explicit MATCH clause does not have the specified effect, and it causes ON DELETE and ON UPDATE clauses to be ignored. Specifying the MATCH should be avoided.

  The MATCH clause in the SQL standard controls how NULL values in a composite (multiple-column) foreign key are handled when comparing to a primary key in the referenced table. MySQL essentially implements the semantics defined by MATCH SIMPLE, which permits a foreign key to be all or partially NULL. In that case, a (child table) row containing such a foreign key can be inserted even though it does not match any row in the referenced (parent) table. (It is possible to implement other semantics using triggers.)

• MySQL requires that the referenced columns be indexed for performance reasons. However, MySQL does not enforce a requirement that the referenced columns be UNIQUE or be declared NOT NULL.

  A FOREIGN KEY constraint that references a non-UNIQUE key is not standard SQL but rather an InnoDB extension. The NDB storage engine, on the other hand, requires an explicit unique key (or primary key) on any column referenced as a foreign key.

  The handling of foreign key references to nonunique keys or keys that contain NULL values is not well defined for operations such as UPDATE or DELETE CASCADE. You are advised to use foreign keys that reference only UNIQUE (including PRIMARY) and NOT NULL keys.

• For storage engines that do not support foreign keys (such as MyISAM), MySQL Server parses and ignores foreign key specifications.

• MySQL parses but ignores “inline REFERENCES specifications” (as defined in the SQL standard) where the references are defined as part of the column specification. MySQL accepts REFERENCES clauses only when specified as part of a separate FOREIGN KEY specification.

  Defining a column to use a REFERENCES tbl_name(col_name) clause has no actual effect and serves only as a memo or comment to you that the column which you are currently defining is intended to refer to a column in another table. It is important to realize when using this syntax that:

  • MySQL does not perform any sort of check to make sure that col_name actually exists in tbl_name (or even that tbl_name itself exists).

  • MySQL does not perform any sort of action on tbl_name such as deleting rows in response to actions taken on rows in the table which you are defining; in other words, this syntax induces no ON DELETE or ON UPDATE behavior whatsoever. (Although you can write an ON DELETE or ON UPDATE clause as part of the REFERENCES clause, it is also ignored.)

  • This syntax creates a column; it does not create any sort of index or key.

  You can use a column so created as a join column, as shown here:

  CREATE TABLE person (
      id SMALLINT UNSIGNED NOT NULL AUTO_INCREMENT,
      name CHAR(60) NOT NULL,
      PRIMARY KEY (id)
  );
  CREATE TABLE shirt (
      id SMALLINT UNSIGNED NOT NULL AUTO_INCREMENT,
      style ENUM('t-shirt', 'polo', 'dress') NOT NULL,
      color ENUM('red', 'blue', 'orange', 'white', 'black') NOT NULL,
      owner SMALLINT UNSIGNED NOT NULL REFERENCES person(id),
      PRIMARY KEY (id)
  );
  INSERT INTO person VALUES (NULL, 'Antonio Paz');
  SELECT @last := LAST_INSERT_ID();
  INSERT INTO shirt VALUES
  (NULL, 'polo', 'blue', @last),
  (NULL, 'dress', 'white', @last),
  (NULL, 't-shirt', 'blue', @last);
  INSERT INTO person VALUES (NULL, 'Lilliana Angelovska');
  SELECT @last := LAST_INSERT_ID();
  INSERT INTO shirt VALUES
  (NULL, 'dress', 'orange', @last),
  (NULL, 'polo', 'red', @last),
  (NULL, 'dress', 'blue', @last),
  (NULL, 't-shirt', 'white', @last);
  SELECT * FROM person;
  +----+---------------------+
  | id | name                |
  +----+---------------------+
  |  1 | Antonio Paz         |
  |  2 | Lilliana Angelovska |
  +----+---------------------+
  SELECT * FROM shirt;
  +----+---------+--------+-------+
  | id | style   | color  | owner |
  +----+---------+--------+-------+
  |  1 | polo    | blue   |     1 |
  |  2 | dress   | white  |     1 |
  |  3 | t-shirt | blue   |     1 |
  |  4 | dress   | orange |     2 |
  |  5 | polo    | red    |     2 |
  |  6 | dress   | blue   |     2 |
  |  7 | t-shirt | white  |     2 |
  +----+---------+--------+-------+
  SELECT s.* FROM person p INNER JOIN shirt s
     ON s.owner = p.id
   WHERE p.name LIKE 'Lilliana%'
     AND s.color <> 'white';
  +----+-------+--------+-------+
  | id | style | color  | owner |
  +----+-------+--------+-------+
  |  4 | dress | orange |     2 |
  |  5 | polo  | red    |     2 |
  |  6 | dress | blue   |     2 |
  +----+-------+--------+-------+

  When used in this fashion, the REFERENCES clause is not displayed in the output of SHOW CREATE TABLE or DESCRIBE:

  SHOW CREATE TABLE shirt\G
  *************************** 1. row ***************************
  Table: shirt
  Create Table: CREATE TABLE `shirt` (
  `id` smallint(5) unsigned NOT NULL auto_increment,
  `style` enum('t-shirt','polo','dress') NOT NULL,
  `color` enum('red','blue','orange','white','black') NOT NULL,
  `owner` smallint(5) unsigned NOT NULL,
  PRIMARY KEY  (`id`)
  ) ENGINE=MyISAM DEFAULT CHARSET=latin1

For information about foreign key constraints, see FOREIGN KEY Constraints.