![]() ![]() USE fkdemo Code language: SQL (Structured Query Language) ( sql ) 1) RESTRICT & NO ACTION actions Let’s create a new database called fkdemo for the demonstration. If you don’t specify the ON DELETE and ON UPDATE clause, the default action is RESTRICT. In fact, MySQL fully supports three actions: RESTRICT, CASCADE and SET NULL. ![]() However, this action is rejected by both InnoDB and NDB tables. SET DEFAULT: is recognized by the MySQL parser.RESTRICT: if a row from the parent table has a matching row in the child table, MySQL rejects deleting or updating rows in the parent table.SET NULL: if a row from the parent table is deleted or updated, the values of the foreign key column (or columns) in the child table are set to NULL.CASCADE: if a row from the parent table is deleted or updated, the values of the matching rows in the child table are automatically deleted or updated.MySQL has five reference options: CASCADE, SET NULL, NO ACTION, RESTRICT, and SET DEFAULT. The reference_option determines the action that MySQL will take when values in the parent key columns are deleted ( ON DELETE) or updated ( ON UPDATE). Third, specify the parent table followed by a list of comma-separated columns to which the foreign key columns reference.įinally, specify how the foreign key maintains the referential integrity between the child and parent tables by using the ON DELETE and ON UPDATE clauses. The foreign key name is also optional and is generated automatically if you skip it. Second, specify a list of comma-separated foreign key columns after the FOREIGN KEY keywords. If you omit the constraint name, MySQL automatically generates a name for the foreign key constraint. Code language: SQL (Structured Query Language) ( sql )įirst, specify the name of the foreign key constraint that you want to create after the CONSTRAINT keyword. Here is the basic syntax of defining a foreign key constraint in the CREATE TABLE or ALTER TABLE statement: įOREIGN KEY (column_name. The foreign key on the column reportTo is known as a recursive or self-referencing foreign key. Each employee reports to zero or one employee and an employee can have zero or many subordinates. This relationship allows the employees table to store the reporting structure between employees and managers. The reportTo column is a foreign key that refers to the employeeNumber column which is the primary key of the employees table. See the following employees table from the sample database. In this case, the foreign key references back to the primary key within the same table. Sometimes, the child and parent tables may refer to the same table. Multiple rows in the orders table can have the same customerNumber. ![]() Once a foreign key constraint is in place, the foreign key columns from the child table must have the corresponding row in the parent key columns of the parent table, or values in these foreign key columns must be NULL (see the SET NULL action example below).įor example, each row in the orders table has a customerNumber that exists in the customerNumber column of the customers table. Typically, the foreign key columns of the child table often refer to the primary key columns of the parent table.Ī table can have more than one foreign key where each foreign key references a primary key of the different parent tables. The customers table is called the parent table or referenced table, and the orders table is known as the child table or referencing table. The customerNumber column in the orders table links to the customerNumber primary key column in the customers table. This relationship is established via the foreign key in the orders table specified by the customerNumber column. The relationship between customers table and orders table is one-to-many. In this diagram, each customer can have zero or many orders and each order belongs to one customer. Let’s take a look at the following customers and orders tables from the sample database. The foreign key places constraints on data in the related tables, which allows MySQL to maintain referential integrity. ![]() Introduction to MySQL foreign keyĪ foreign key is a column or group of columns in a table that links to a column or group of columns in another table. Summary: in this tutorial, you will learn about MySQL foreign key and how to create, drop, and disable a foreign key constraint. ![]()
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