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:
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 theCONSTRAINT
keyword, a constraint name name is generated automatically.If the
CONSTRAINT
symbol
clause is not defined, or a symbol is not included following theCONSTRAINT
keyword, bothInnoDB
andNDB
storage engines ignoreFOREIGN_KEY
.index_name
The
CONSTRAINT
value, if defined, must be unique in the database. A duplicatesymbol
symbol
results in an error similar to: ERROR 1005 (HY000): Can't create table 'test.fk1' (errno: 121).NDB Cluster stores foreign key names using the same lettercase with which they are created.
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.
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
REFERENCES
privilege 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
INTEGER
andDECIMAL
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.Previously,
InnoDB
allowed a foreign key to reference any index column or group of columns, even a non-unique index or partial index, an extension of standard SQL. This is still allowed for backwards compatibility, but is now deprecated; in addition, it must be enabled by settingrestrict_fk_on_non_standard_key
. If this is done, there must still be an index in the referenced table where the referenced columns are the first columns in the same order. Hidden columns thatInnoDB
adds to an index are also considered in such cases (see Section 17.6.2.1, “Clustered and Secondary Indexes”). You should expect support for use of nonstandard keys to be removed in a future version of MySQL, and migrate away from their use.NDB
always requires an explicit unique key (or primary key) on any column referenced as a foreign key.Index prefixes on foreign key columns are not supported. Consequently,
BLOB
andTEXT
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.This restriction does not apply for
NDB
tables that are partitioned byKEY
orLINEAR KEY
(the only user partitioning types supported by theNDB
storage 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 1.7.2.3, “FOREIGN KEY Constraint Differences”.
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. BothON DELETE CASCADE
andON UPDATE CASCADE
are supported. Between two tables, do not define severalON UPDATE CASCADE
clauses that act on the same column in the parent table or in the child table.If a
FOREIGN KEY
clause is defined on both tables in a foreign key relationship, making both tables a parent and child, anON UPDATE CASCADE
orON DELETE CASCADE
subclause defined for oneFOREIGN KEY
clause must be defined for the other in order for cascading operations to succeed. If anON UPDATE CASCADE
orON DELETE CASCADE
subclause is only defined for oneFOREIGN KEY
clause, cascading operations fail with an error.NoteCascaded 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 toNULL
. BothON DELETE SET NULL
andON 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 asNOT NULL
.RESTRICT
: Rejects the delete or update operation for the parent table. SpecifyingRESTRICT
(orNO ACTION
) is the same as omitting theON DELETE
orON UPDATE
clause.NO ACTION
: A keyword from standard SQL. ForInnoDB
, this is equivalent toRESTRICT
; the delete or update operation for the parent table is immediately rejected if there is a related foreign key value in the referenced table.NDB
supports deferred checks, andNO ACTION
specifies a deferred check; when this is used, constraint checks are not performed until commit time. Note that forNDB
tables, this causes all foreign key checks made for both parent and child tables to be deferred.SET DEFAULT
: This action is recognized by the MySQL parser, but bothInnoDB
andNDB
reject table definitions containingON DELETE SET DEFAULT
orON 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 NO ACTION
.
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 SHOW
CREATE TABLE
output and in tables dumped with
mysqldump.
For NDB
tables, ON
UPDATE CASCADE
is not supported where the reference
is to the parent table's primary key.
For NDB
tables, ON
DELETE CASCADE
is not supported where the child
table contains one or more columns of any of the
TEXT
or
BLOB
types. (Bug #89511, Bug
#27484882)
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
use CASCADE
, SET NULL
,
or SET DEFAULT
as ON
UPDATE
referential actions, nor can it use
SET NULL
or SET DEFAULT
as ON DELETE
referential actions.
A foreign key constraint on the base column of a stored
generated column cannot use CASCADE
,
SET NULL
, or SET DEFAULT
as ON UPDATE
or ON
DELETE
referential actions.
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;
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 first create an index on the column(s) referenced by the
foreign key.
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 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
supported for
ALTER TABLE ...
ALGORITHM=INPLACE
. It is not supported for
ALTER TABLE ...
ALGORITHM=COPY
.
In MySQL, InnoDB and NDB tables support checking of foreign
key constraints. 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
has the same effect on NDB
tables
as it does for InnoDB
tables.
The foreign_key_checks
variable is dynamic and supports both global and session
scopes. For information about using system variables, see
Section 7.1.9, “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_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. Disablingforeign_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 whileforeign_key_checks
is disabled are not checked for consistency whenforeign_key_checks
is re-enabled.
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 LOCK
TABLES
, any tables related by a foreign key
constraint are opened and locked implicitly. For foreign key
checks, a shared read-only lock
(LOCK TABLES
READ
) is taken on related tables. For cascading
updates, a shared-nothing write lock
(LOCK TABLES
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
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 |
+--------------+------------+-------------+-----------------+
You can obtain information specific to
InnoDB
foreign keys from the
INNODB_FOREIGN
and
INNODB_FOREIGN_COLS
tables.
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
SHOW ENGINE
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,
ER_NO_REFERENCED_ROW_2
and
ER_ROW_IS_REFERENCED_2
error
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
(ER_NO_REFERENCED_ROW
and
ER_ROW_IS_REFERENCED
).
An exception is that, for stored programs defined to execute
with 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.