Movatterモバイル変換

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 aRESTRICT type constraint, and there is a child row with several parent rows,InnoDB does not permit the deletion of any of the parent rows.

IfON UPDATE CASCADE orON UPDATE SET NULL recurses to update thesame table it has previously updated during the same cascade, it acts likeRESTRICT. This means that you cannot use self-referentialON UPDATE CASCADE orON UPDATE SET NULL operations. This is to prevent infinite loops resulting from cascaded updates. A self-referentialON DELETE SET NULL, on the other hand, is possible, as is a self-referentialON 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 theentire 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, includingInnoDB, recognizes or enforces theMATCH clause used in referential-integrity constraint definitions. Use of an explicitMATCH clause does not have the specified effect, and it causesON DELETE andON UPDATE clauses to be ignored. Specifying theMATCH should be avoided.

TheMATCH clause in the SQL standard controls howNULL 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 byMATCH SIMPLE, which permits a foreign key to be all or partiallyNULL. 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 beUNIQUE or be declaredNOT NULL.

AFOREIGN KEY constraint that references a non-UNIQUE key is not standard SQL but rather anInnoDB extension. TheNDB 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 containNULL values is not well defined for operations such asUPDATE orDELETE CASCADE. You are advised to use foreign keys that reference onlyUNIQUE (includingPRIMARY) andNOT NULL keys.

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

MySQL parses but ignores“inlineREFERENCES specifications” (as defined in the SQL standard) where the references are defined as part of the column specification. MySQL acceptsREFERENCES clauses only when specified as part of a separateFOREIGN KEY specification.

Defining a column to use aREFERENCEStbl_name(col_name) clause has no actual effect andserves 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:

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, theREFERENCES clause is not displayed in the output ofSHOW CREATE TABLE orDESCRIBE:

SHOW CREATE TABLE shirt\G*************************** 1. row ***************************Table: shirtCreate 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=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci