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13.1.17.6 FOREIGN KEY Constraints

MySQL supports foreign keys, which permit cross-referencing related data across tables, and foreign key constraints, which help keep the related data consistent.

A foreign key relationship involves a parent table that holds the initial column values, and a child table with column values that reference the parent column values. A foreign key constraint is defined on the child table.

The essential syntax for a defining a foreign key constraint in a CREATE TABLE or ALTER TABLE statement includes the following:

[CONSTRAINT [symbol]] FOREIGN KEY
    [index_name] (col_name, ...)
    REFERENCES tbl_name (col_name,...)
    [ON DELETE reference_option]
    [ON UPDATE reference_option]

reference_option:
    RESTRICT | CASCADE | SET NULL | NO ACTION | SET DEFAULT

Foreign key constraint usage is described under the following topics in this section:

Identifiers

Foreign key constraint naming is governed by the following rules:

  • The CONSTRAINT symbol value is used, if defined.

  • If the CONSTRAINT symbol clause is not defined, or a symbol is not included following the CONSTRAINT keyword, a constraint name name is generated automatically.

  • The CONSTRAINT symbol value, if defined, must be unique in the database. A duplicate symbol results in an error similar to: ERROR 1005 (HY000): Can't create table 'test.fk1' (errno: 121).

Table and column identifiers in a FOREIGN KEY ... REFERENCES clause can be quoted within backticks (`). Alternatively, double quotation marks (") can be used if the ANSI_QUOTES SQL mode is enabled. The lower_case_table_names system variable setting is also taken into account.

Conditions and Restrictions

Foreign key constraints are subject to the following conditions and restrictions:

  • Parent and child tables must use the same storage engine, and they cannot be defined as temporary tables.

  • Creating a foreign key constraint requires at least one of the SELECT, INSERT, UPDATE, DELETE, or REFERENCES privileges on the parent table as of 5.5.41.

  • Corresponding columns in the foreign key and the referenced key must have similar data types. The size and sign of integer types must be the same. The length of string types need not be the same. For nonbinary (character) string columns, the character set and collation must be the same.

  • MySQL supports foreign key references between one column and another within a table. (A column cannot have a foreign key reference to itself.) In these cases, a child table record refers to a dependent record within the same table.

  • MySQL requires indexes on foreign keys and referenced keys so that foreign key checks can be fast and not require a table scan. In the referencing table, there must be an index where the foreign key columns are listed as the first columns in the same order. Such an index is created on the referencing table automatically if it does not exist. This index might be silently dropped later if you create another index that can be used to enforce the foreign key constraint. index_name, if given, is used as described previously.

  • InnoDB permits a foreign key to reference any index column or group of columns. However, in the referenced table, there must be an index where the referenced columns are the first columns in the same order. Hidden columns that InnoDB adds to an index are also considered (see Section 14.9.2.1, “Clustered and Secondary Indexes”).

    InnoDB does not require an explicit unique key (or primary key) on any column referenced as a foreign key, which is an extension of standard SQL.

  • Index prefixes on foreign key columns are not supported. Consequently, BLOB and TEXT columns cannot be included in a foreign key because indexes on those columns must always include a prefix length.

  • InnoDB does not currently support foreign keys for tables with user-defined partitioning. This includes both parent and child tables.

  • A table in a foreign key relationship cannot be altered to use another storage engine. To change the storage engine, you must drop any foreign key constraints first.

For information about how the MySQL implementation of foreign key constraints differs from the SQL standard, see Section 1.7.2.3, “FOREIGN KEY Constraint Differences”.

Referential Actions

When an UPDATE or DELETE operation affects a key value in the parent table that has matching rows in the child table, the result depends on the referential action specified by ON UPDATE and ON DELETE subclauses of the FOREIGN KEY clause. Referential actions include:

  • CASCADE: Delete or update the row from the parent table and automatically delete or update the matching rows in the child table. Both ON DELETE CASCADE and ON UPDATE CASCADE are supported. Between two tables, do not define several ON UPDATE CASCADE clauses that act on the same column in the parent table or in the child table.

    Note

    Cascaded foreign key actions do not activate triggers.

  • SET NULL: Delete or update the row from the parent table and set the foreign key column or columns in the child table to NULL. Both ON DELETE SET NULL and ON UPDATE SET NULL clauses are supported.

    If you specify a SET NULL action, make sure that you have not declared the columns in the child table as NOT NULL.

  • RESTRICT: Rejects the delete or update operation for the parent table. Specifying RESTRICT (or NO ACTION) is the same as omitting the ON DELETE or ON UPDATE clause.

  • NO ACTION: A keyword from standard SQL. In MySQL, equivalent to RESTRICT. The MySQL Server rejects the delete or update operation for the parent table if there is a related foreign key value in the referenced table. Some database systems have deferred checks, and NO ACTION is a deferred check. In MySQL, foreign key constraints are checked immediately, so NO ACTION is the same as RESTRICT.

  • SET DEFAULT: This action is recognized by the MySQL parser, but InnoDB rejects table definitions containing ON DELETE SET DEFAULT or ON UPDATE SET DEFAULT clauses.

For storage engines that support foreign keys, MySQL rejects any INSERT or UPDATE operation that attempts to create a foreign key value in a child table if there is no matching candidate key value in the parent table.

For an ON DELETE or ON UPDATE that is not specified, the default action is always RESTRICT.

InnoDB performs cascading operations using a depth-first search algorithm on the records of the index that corresponds to the foreign key constraint.

Foreign Key Constraint Examples

This simple example relates parent and child tables through a single-column foreign key:

CREATE TABLE parent (
    id INT NOT NULL,
    PRIMARY KEY (id)
) ENGINE=INNODB;

CREATE TABLE child (
    id INT,
    parent_id INT,
    INDEX par_ind (parent_id),
    FOREIGN KEY (parent_id)
        REFERENCES parent(id)
        ON DELETE CASCADE
) ENGINE=INNODB;

This is a more complex example in which a product_order table has foreign keys for two other tables. One foreign key references a two-column index in the product table. The other references a single-column index in the customer table:

CREATE TABLE product (
    category INT NOT NULL, id INT NOT NULL,
    price DECIMAL,
    PRIMARY KEY(category, id)
)   ENGINE=INNODB;

CREATE TABLE customer (
    id INT NOT NULL,
    PRIMARY KEY (id)
)   ENGINE=INNODB;

CREATE TABLE product_order (
    no INT NOT NULL AUTO_INCREMENT,
    product_category INT NOT NULL,
    product_id INT NOT NULL,
    customer_id INT NOT NULL,

    PRIMARY KEY(no),
    INDEX (product_category, product_id),
    INDEX (customer_id),

    FOREIGN KEY (product_category, product_id)
      REFERENCES product(category, id)
      ON UPDATE CASCADE ON DELETE RESTRICT,

    FOREIGN KEY (customer_id)
      REFERENCES customer(id)
)   ENGINE=INNODB;
Adding Foreign Key Constraints

You can add a foreign key constraint to an existing table using the following ALTER TABLE syntax:

ALTER TABLE tbl_name
    ADD [CONSTRAINT [symbol]] FOREIGN KEY
    [index_name] (col_name, ...)
    REFERENCES tbl_name (col_name,...)
    [ON DELETE reference_option]
    [ON UPDATE reference_option]

The foreign key can be self referential (referring to the same table). When you add a foreign key constraint to a table using ALTER TABLE, remember to create the required indexes first.

Dropping Foreign Key Constraints

You can drop a foreign key constraint using the following ALTER TABLE syntax:

ALTER TABLE tbl_name DROP FOREIGN KEY fk_symbol;

If the FOREIGN KEY clause defined a CONSTRAINT name when you created the constraint, you can refer to that name to drop the foreign key constraint. Otherwise, a constraint name was generated internally, and you must use that value. To determine the foreign key constraint name, use SHOW CREATE TABLE:

mysql> SHOW CREATE TABLE child\G
*************************** 1. row ***************************
       Table: child
Create Table: CREATE TABLE `child` (
  `id` int(11) DEFAULT NULL,
  `parent_id` int(11) DEFAULT NULL,
  KEY `par_ind` (`parent_id`),
  CONSTRAINT `child_ibfk_1` FOREIGN KEY (`parent_id`) 
  REFERENCES `parent` (`id`) ON DELETE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=latin1

mysql> ALTER TABLE child DROP FOREIGN KEY `child_ibfk_1`;

Adding and dropping a foreign key in separate clauses of a single ALTER TABLE statement may be problematic in some cases and is therefore unsupported. Use separate statements for each operation.

If an ALTER TABLE statement results in changes to column values (for example, because a column is truncated), MySQL's foreign key constraint checks do not notice possible violations caused by changing the values.

Foreign Key Checks

Foreign key checking is controlled by the foreign_key_checks variable, which is enabled by default. Typically, you leave this variable enabled during normal operation to enforce referential integrity.

The foreign_key_checks variable is dynamic and supports both global and session scopes. For information about using system variables, see Section 5.1.8, “Using System Variables”.

Disabling foreign key checking is useful when:

  • Dropping a table that is referenced by a foreign key constraint. A referenced table can only be dropped after foreign_key_checks is disabled. When you drop a table, constraints defined on the table are also dropped.

  • Reloading tables in different order than required by their foreign key relationships. For example, mysqldump produces correct definitions of tables in the dump file, including foreign key constraints for child tables. To make it easier to reload dump files for tables with foreign key relationships, mysqldump automatically includes a statement in the dump output that disables foreign_key_checks. This enables you to import the tables in any order in case the dump file contains tables that are not correctly ordered for foreign keys. Disabling foreign_key_checks also speeds up the import operation by avoiding foreign key checks.

  • Executing LOAD DATA operations, to avoid foreign key checking.

  • Performing an ALTER TABLE operation on a table that has a foreign key relationship.

When foreign_key_checks is disabled, foreign key constraints are ignored, with the following exceptions:

  • Recreating a table that was previously dropped returns an error if the table definition does not conform to the foreign key constraints that reference the table. The table must have the correct column names and types. It must also have indexes on the referenced keys. If these requirements are not satisfied, MySQL returns Error 1005 that refers to errno: 150 in the error message, which means that a foreign key constraint was not correctly formed.

  • Altering a table returns an error (errno: 150) if a foreign key definition is incorrectly formed for the altered table.

  • Dropping an index required by a foreign key constraint. The foreign key constraint must be removed before dropping the index.

  • Creating a foreign key constraint where a column references a nonmatching column type.

Disabling foreign_key_checks has these additional implications:

  • It is permitted to drop a database that contains tables with foreign keys that are referenced by tables outside the database.

  • It is permitted to drop a table with foreign keys referenced by other tables.

  • Enabling foreign_key_checks does not trigger a scan of table data, which means that rows added to a table while foreign_key_checks is disabled are not checked for consistency when foreign_key_checks is re-enabled.

Foreign Key Definitions and Metadata

To view a foreign key definition, use SHOW CREATE TABLE:

mysql> SHOW CREATE TABLE child\G
*************************** 1. row ***************************
       Table: child
Create Table: CREATE TABLE `child` (
  `id` int(11) DEFAULT NULL,
  `parent_id` int(11) DEFAULT NULL,
  KEY `par_ind` (`parent_id`),
  CONSTRAINT `child_ibfk_1` FOREIGN KEY (`parent_id`) 
  REFERENCES `parent` (`id`) ON DELETE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=latin1

You can obtain information about foreign keys from the INFORMATION_SCHEMA.KEY_COLUMN_USAGE table. An example of a query against this table is shown here:

mysql> SELECT TABLE_SCHEMA, TABLE_NAME, COLUMN_NAME, CONSTRAINT_NAME 
       FROM INFORMATION_SCHEMA.KEY_COLUMN_USAGE 
       WHERE REFERENCED_TABLE_SCHEMA IS NOT NULL; 
+--------------+------------+-------------+-----------------+
| TABLE_SCHEMA | TABLE_NAME | COLUMN_NAME | CONSTRAINT_NAME |
+--------------+------------+-------------+-----------------+
| test         | child      | parent_id   | child_ibfk_1    |
+--------------+------------+-------------+-----------------+
Foreign Key Errors

In the event of a foreign key error involving InnoDB tables (usually Error 150 in the MySQL Server), information about the latest foreign key error can be obtained by checking SHOW ENGINE INNODB STATUS output.

mysql> SHOW ENGINE INNODB STATUS\G
...
------------------------
LATEST FOREIGN KEY ERROR
------------------------
141016 15:37:30 Transaction:
TRANSACTION 3D005, ACTIVE 0 sec inserting
mysql tables in use 1, locked 1
4 lock struct(s), heap size 1248, 3 row lock(s), undo log entries 3
MySQL thread id 1, OS thread handle 0x7f0ee440e700, query id 70 localhost root
update
INSERT INTO child VALUES
    (NULL, 1)
    , (NULL, 2)
    , (NULL, 3)
    , (NULL, 4)
    , (NULL, 5)
    , (NULL, 6)
Foreign key constraint fails for table `mysql`.`child`:
,
  CONSTRAINT `child_ibfk_1` FOREIGN KEY (`parent_id`) REFERENCES `parent` (`id`)
  ON DELETE CASCADE ON UPDATE CASCADE
Trying to add in child table, in index `par_ind` tuple:
DATA TUPLE: 2 fields;
 0: len 4; hex 80000003; asc     ;;
 1: len 4; hex 80000003; asc     ;;

But in parent table `mysql`.`parent`, in index `PRIMARY`,
the closest match we can find is record:
PHYSICAL RECORD: n_fields 3; compact format; info bits 0
 0: len 4; hex 80000004; asc     ;;
 1: len 6; hex 00000003d002; asc       ;;
 2: len 7; hex 8300001d480137; asc     H 7;;
...
Warning

ER_NO_REFERENCED_ROW_2 and ER_ROW_IS_REFERENCED_2 error messages for foreign key operations expose information about parent tables, even if the user has no parent table access privileges. To hide information about parent tables, include the appropriate condition handlers in application code and stored programs.