FOREIGN KEY constraints

InnoDB supports foreign key constraints. The syntax for a foreign key constraint definition in InnoDB looks like this:

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

reference_option:
    RESTRICT | CASCADE | SET NULL | NO ACTION

CHECK constraint expressions

MariaDB starting with 10.2.1

MariaDB 10.2.1 now enforces constraints. Before MariaDB 10.2.1 constraint expressions were accepted in the syntax but ignored.

In MariaDB 10.2.1 you can define constraints in 2 different ways:

  • CHECK(expression) given as part of a column definition.
  • CONSTRAINT [constraint_name] CHECK (expression)

Before a row is inserted or updated, all constraints are evaluated in the order they are defined. If any constraints fails, then the row will not be inserted or updated. One can use most deterministic functions in a constraint, including UDFs.

CREATE TABLE t1 (a INT CHECK (a>2), b INT CHECK (b>2), CONSTRAINT a_greater CHECK (a>b));

If you use the second format and you don't give a name to the constraint, then the constraint will get an automatically generated name. This is done so that you can later delete the constraint with ALTER TABLE DROP constraint_name. Constraints cannot be added or changed, only dropped.

One can disable all constraint expression checks by setting the check_constraint_checks variable to OFF. This is useful for example when loading a table that violates some constraints that you want to later find and fix in SQL.

Replication

In row-based replication, only the master checks constraints, and failed statements will not be replicated. In statement-based replication, the slaves will also check constraints. Constraints should therefore be identical, as well as deterministic, in a replication environment.

Examples

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),
                            FOREIGN KEY (product_category, product_id)
                              REFERENCES product(category, id)
                              ON UPDATE CASCADE ON DELETE RESTRICT,
                            INDEX (customer_id),
                            FOREIGN KEY (customer_id)
                              REFERENCES customer(id)) ENGINE=INNODB;
MariaDB starting with 10.2.1

The following examples will work from MariaDB 10.2.1 onwards.

Numeric constraints and comparisons:

CREATE TABLE t1 (a INT CHECK (a>2), b INT CHECK (b>2), CONSTRAINT a_greater CHECK (a>b));

INSERT INTO t1(a) VALUES (1);
ERROR 4022 (23000): CONSTRAINT `a` failed for `test`.`t1`

INSERT INTO t1(a,b) VALUES (3,4);
ERROR 4022 (23000): CONSTRAINT `a_greater` failed for `test`.`t1`

INSERT INTO t1(a,b) VALUES (4,3);
Query OK, 1 row affected (0.04 sec)

Dropping a constraint:

ALTER TABLE t1 DROP CONSTRAINT a_greater;

Date comparisons and character length:

CREATE TABLE t2 (name VARCHAR(30) CHECK (CHAR_LENGTH(name)>2), start_date DATE, 
  end_date DATE CHECK (start_date IS NULL OR end_date IS NULL OR start_date<end_date));

INSERT INTO t2(name, start_date, end_date) VALUES('Ione', '2003-12-15', '2014-11-09');
Query OK, 1 row affected (0.04 sec)

INSERT INTO t2(name, start_date, end_date) VALUES('Io', '2003-12-15', '2014-11-09');
ERROR 4022 (23000): CONSTRAINT `name` failed for `test`.`t2`

INSERT INTO t2(name, start_date, end_date) VALUES('Ione', NULL, '2014-11-09');
Query OK, 1 row affected (0.04 sec)

INSERT INTO t2(name, start_date, end_date) VALUES('Ione', '2015-12-15', '2014-11-09');
ERROR 4022 (23000): CONSTRAINT `end_date` failed for `test`.`t2`

A misplaced parenthesis:

CREATE TABLE t3 (name VARCHAR(30) CHECK (CHAR_LENGTH(name>2)), start_date DATE, 
  end_date DATE CHECK (start_date IS NULL OR end_date IS NULL OR start_date<end_date));
Query OK, 0 rows affected (0.32 sec)

INSERT INTO t3(name, start_date, end_date) VALUES('Io', '2003-12-15', '2014-11-09');
Query OK, 1 row affected, 1 warning (0.04 sec)

SHOW WARNINGS;
+---------+------+----------------------------------------+
| Level   | Code | Message                                |
+---------+------+----------------------------------------+
| Warning | 1292 | Truncated incorrect DOUBLE value: 'Io' |
+---------+------+----------------------------------------+

Compare the definition of table t2 to table t3. CHAR_LENGTH(name)>2 is very different to CHAR_LENGTH(name>2) as the latter mistakenly performs a numeric comparison on the name field, leading to unexpected results.

See also

Comments

Comments loading...