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
CREATE TABLE or
ALTER TABLE statement includes
[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:
Foreign key constraint naming is governed by the following rules:
symbolvalue is used, if defined.
symbolclause is not defined, or a symbol is not included following the
CONSTRAINTkeyword, a constraint name name is generated automatically.
Prior to MySQL 8.0.16, if the
symbolclause was not defined, or a symbol was not included following the
NDBstorage engines would use the
FOREIGN_KEYif defined. In MySQL 8.0.16 and higher, the
CONSTRAINTvalue, if defined, must be unique in the database. A duplicate
symbolresults in an error similar to: ERROR 1005 (HY000): Can't create table 'test.fk1' (errno: 121).
NDB Cluster stores foreign names using the same lettercase with which they are created. Prior to version 8.0.20, when processing
SELECTand other SQL statements,
NDBcompared the names of foreign keys in such statements with the names as stored in a case-sensitive fashion when
lower_case_table_nameswas equal to 0. In NDB 8.0.20 and later, this value no longer has any effect on how such comparisons are made, and they are always done without regard to lettercase. (Bug #30512043)
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
variable setting is also taken into account.
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 the
REFERENCESprivilege on the parent table.
Corresponding columns in the foreign key and the referenced key must have similar data types. The size and sign of fixed precision types such as
DECIMALmust 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.
InnoDBpermits 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
InnoDBadds to an index are also considered (see Section 18.104.22.168, “Clustered and Secondary Indexes”).
NDBrequires an explicit unique key (or primary key) on any column referenced as a foreign key.
InnoDBdoes not, which is an extension of standard SQL.
Index prefixes on foreign key columns are not supported. Consequently,
TEXTcolumns cannot be included in a foreign key because indexes on those columns must always include a prefix length.
InnoDBdoes not currently support foreign keys for tables with user-defined partitioning. This includes both parent and child tables.
This restriction does not apply for
NDBtables that are partitioned by
LINEAR KEY(the only user partitioning types supported by the
NDBstorage engine); these may have foreign key references or be the targets of such references.
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.
A foreign key constraint cannot reference a virtual generated column.
For information about how the MySQL implementation of foreign key constraints differs from the SQL standard, see Section 22.214.171.124, “FOREIGN KEY Constraint Differences”.
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 DELETE subclauses of the
FOREIGN KEY clause. Referential actions
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 CASCADEand
ON UPDATE CASCADEare supported. Between two tables, do not define several
ON UPDATE CASCADEclauses that act on the same column in the parent table or in the child table.
FOREIGN KEYclause is defined on both tables in a foreign key relationship, making both tables a parent and child, an
ON UPDATE CASCADEor
ON DELETE CASCADEsubclause defined for one
FOREIGN KEYclause must be defined for the other in order for cascading operations to succeed. If an
ON UPDATE CASCADEor
ON DELETE CASCADEsubclause is only defined for one
FOREIGN KEYclause, cascading operations fail with an error.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
ON DELETE SET NULLand
ON UPDATE SET NULLclauses are supported.
If you specify a
SET NULLaction, make sure that you have not declared the columns in the child table as
RESTRICT: Rejects the delete or update operation for the parent table. Specifying
NO ACTION) is the same as omitting the
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 ACTIONis a deferred check. In MySQL, foreign key constraints are checked immediately, so
NO ACTIONis the same as
SET DEFAULT: This action is recognized by the MySQL parser, but both
NDBreject table definitions containing
ON DELETE SET DEFAULTor
ON UPDATE SET DEFAULTclauses.
For storage engines that support foreign keys, MySQL rejects
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.
ON DELETE or
UPDATE that is not specified, the default action is
As the default, an
ON DELETE NO ACTION or
ON UPDATE NO ACTION clause that is
specified explicitly does not appear in
SHOW CREATE TABLE output or in
tables dumped with mysqldump.
RESTRICT, which is an equivalent
non-default keyword, appears in
CREATE TABLE output and in tables dumped with
UPDATE CASCADE is not supported where the reference
is to the parent table's primary key.
As of NDB 8.0.16: For
ON DELETE CASCADE is not supported where
the child table contains one or more columns of any of the
BLOB types. (Bug #89511, Bug
InnoDB performs cascading operations using
a depth-first search algorithm on the records of the index
that corresponds to the foreign key constraint.
A foreign key constraint on a stored generated column cannot
SET DEFAULT as
UPDATE referential actions, nor can it use
SET NULL or
ON DELETE referential actions.
A foreign key constraint on the base column of a stored
generated column cannot use
SET NULL, or
ON UPDATE or
DELETE referential actions.
This simple example relates
child tables through a single-column
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
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;
You can add a foreign key constraint to an existing table
using the following
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 first create an index on the column(s) referenced by the
You can drop a foreign key constraint using the following
ALTER TABLE syntax:
ALTER TABLE tbl_name DROP FOREIGN KEY fk_symbol;
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
mysql> SHOW CREATE TABLE child\G *************************** 1. row *************************** Table: child Create Table: CREATE TABLE `child` ( `id` int DEFAULT NULL, `parent_id` int 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=utf8mb4 COLLATE=utf8mb4_0900_ai_ci mysql> ALTER TABLE child DROP FOREIGN KEY `child_ibfk_1`;
Adding and dropping a foreign key in the same
ALTER TABLE statement is
ALTER TABLE ...
ALGORITHM=INPLACE. It is not supported for
ALTER TABLE ...
In MySQL, InnoDB and NDB tables support checking of foreign
key constraints. Foreign key checking is controlled by the
which is enabled by default. Typically, you leave this
variable enabled during normal operation to enforce
referential integrity. The
has the same effect on
as it does for
variable is dynamic and supports both global and session
scopes. For information about using system variables, see
Section 5.1.9, “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_checksis 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_checksalso speeds up the import operation by avoiding foreign key checks.
LOAD DATAoperations, to avoid foreign key checking.
ALTER TABLEoperation on a table that has a foreign key relationship.
disabled, foreign key constraints are ignored, with the
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.
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.
foreign_key_checksdoes not trigger a scan of table data, which means that rows added to a table while
foreign_key_checksis disabled are not checked for consistency when
MySQL extends metadata locks, as necessary, to tables that are related by a foreign key constraint. Extending metadata locks prevents conflicting DML and DDL operations from executing concurrently on related tables. This feature also enables updates to foreign key metadata when a parent table is modified. In earlier MySQL releases, foreign key metadata, which is owned by the child table, could not be updated safely.
If a table is locked explicitly with
TABLES, any tables related by a foreign key
constraint are opened and locked implicitly. For foreign key
checks, a shared read-only lock
READ) is taken on related tables. For cascading
updates, a shared-nothing write lock
WRITE) is taken on related tables that are involved
in the operation.
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 DEFAULT NULL, `parent_id` int 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=utf8mb4 COLLATE=utf8mb4_0900_ai_ci
You can obtain information about foreign keys from the
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 | +--------------+------------+-------------+-----------------+
You can obtain information specific to
InnoDB foreign keys from the
Example queries are show here:
mysql> SELECT * FROM INFORMATION_SCHEMA.INNODB_FOREIGN \G *************************** 1. row *************************** ID: test/child_ibfk_1 FOR_NAME: test/child REF_NAME: test/parent N_COLS: 1 TYPE: 1 mysql> SELECT * FROM INFORMATION_SCHEMA.INNODB_FOREIGN_COLS \G *************************** 1. row *************************** ID: test/child_ibfk_1 FOR_COL_NAME: parent_id REF_COL_NAME: id POS: 0
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
INNODB STATUS output.
mysql> SHOW ENGINE INNODB STATUS\G ... ------------------------ LATEST FOREIGN KEY ERROR ------------------------ 2018-04-12 14:57:24 0x7f97a9c91700 Transaction: TRANSACTION 7717, ACTIVE 0 sec inserting mysql tables in use 1, locked 1 4 lock struct(s), heap size 1136, 3 row lock(s), undo log entries 3 MySQL thread id 8, OS thread handle 140289365317376, query id 14 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 `test`.`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 `test`.`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 000000001e19; asc ;; 2: len 7; hex 81000001110137; asc 7;; ...
If a user has table-level privileges for all parent tables,
messages for foreign key operations expose information about
parent tables. If a user does not have table-level
privileges for all parent tables, more generic error
messages are displayed instead
An exception is that, for stored programs defined to execute
DEFINER privileges, the user against
which privileges are assessed is the user in the program
DEFINER clause, not the invoking user. If
that user has table-level parent table privileges, parent
table information is still displayed. In this case, it is
the responsibility of the stored program creator to hide the
information by including appropriate condition handlers.