MySQL - Foreign Key

In this tutorial, you will learn about MySQL foreign key and how to create, add, and drop foreign key constraints in MySQL.
 
Introduction to MySQL foreign key
 
A foreign key is a field in a table that matches a field of another table. A foreign key places constraints on data in the related tables that, which enables MySQL to maintain referential integrity.
 
We have two tables: customers and orders. Each customer has zero or more orders and each order belongs to only one customer. The relationship between customers table and orders table is one-to-many, and it is established by a foreign key in the orders table specified by thecustomerNumber field. The customerNumber field in the orders table relates to thecustomerNumber primary key field in customers table.
 
The customers table is called parent table or referenced table, and the orders table is known aschild table or referencing table.
A foreign key has not only one column but also a set of columns. The columns in the child table often refer to the primary key columns in the parent table.
 
A table may have more than one foreign key, and each foreign key in the child table can have a different parent table.
A row in the child table must contain values that exist in the parent table e.g., each order record in theorders table must have a customerNumber that exists in the customers table. Multiple orders can refer to the same customer therefore this relationship is called one (customer) to many (orders), or one-to-many.
 
Sometimes, the child and parent table is the same table. The foreign key refers back to the primary key ofthe table e.g., the following employees table :
 
The reportTo column is a foreign key that refers to the employeeNumber column which is theprimary key of the employees table to reflect the reporting structure between employees i.e., each employee reports to anther employee and an employee can have zero or more direct reports.
 
The reportTo foreign key is also known as recursive or self-referencing foreign key.
 
Foreign keys enforce referential integrity that helps you maintain the consistency and integrity of the data automatically. For example, you cannot create an order for a non-existent customer.
 
In addition, you can set up a cascade on delete action for the customerNumber foreign key so that when you delete a customer in the customers table, all the orders associated with the customer are also deleted. This saves you time and efforts of using multiple
DELETE statements or a DELETE JOIN statement.
 
The same as deletion, you can also define a cascade on update action for the customerNumberforeign key to perform cross-table update without using multiple UPDATE statements or an UPDATE JOIN statement.
 
In MySQL, the InnoDB storage engine supports foreign keys so that you must create InnoDB tables in order to use foreign key constraints.
 
MySQL create table foreign key
 
MySQL create foreign key syntax
 
The following syntax illustrates how to define a foreign key in a child table in CREATE TABLE statement.
 
CONSTRAINT constraint_name
FOREIGN KEY foreign_key_name (columns)
REFERENCES parent_table(columns)
ON DELETE action
ON UPDATE action
 
Let’s examine the syntax in greater detail:
  -  The CONSTRAINT clause allows you to define constraint name for the foreign key constraint. If you omit it, MySQL will generate a name automatically.
 
  -  The FOREIGN KEY clause specifies the columns in the child table that refer to primary keycolumns in the parent table. You can put a foreign key name after FOREIGN KEY clause or leave it to let MySQL to create a name for you. Notice that MySQL automatically creates an index with theforeign_key_name name.
 
  -  The REFERENCES clause specifies the parent table and its columns to which the columns in the child table refer. The number of columns in child table and parent table specified in the FOREIGN KEY and REFERENCES must be the same.
 
  -  The ON DELETE clause allows you to define what happens to the records in the child table when the records in the parent table are deleted. If you omit the ON DELETE clause and delete a record in the parent table that has records in the child table refer to, MySQL will reject the deletion. In addition, MySQL also provides you with actions so that you can have other options such as ON DELETE CASCADE that lets MySQL to delete records in the child table that refer to a record in the parent table when the record in the parent table is deleted. If you don’t want the related records in the child table to be deleted, you use the ON DELETE SET NULL action instead. MySQL will set the foreign key column values in the child table to NULL when the record in the parent table is deleted, with a condition that the foreign key column in the child table must accept NULL values. Notice that if you use ON DELETE NO ACTION or ON DELETE RESTRICT action, MySQL will reject the deletion.
 
  -  The ON UPDATE clause enables you to specify what happens to the rows in the child table when rows in the parent table are updated. You can omit the ON UPDATE clause to let MySQL to reject any update to the rows in the child table when the rows in the parent table are updated. The ON UPDATE CASCADE action allows you to perform cross-table update, and the ON UPDATE SET NULL action resets the values in the rows in the child table to NULL values when the rows in the parent table are updated. The ON UPDATE NO ACTION or UPDATE RESTRICT actions reject any updates.
 
MySQL create table foreign key example
 
The following example creates a dbdemo database and two tables: categories and products. Each category has one or more products and each product belongs to only one category. The cat_idfield in the products table is defined as a foreign key with
UPDATE ON CASCADE and DELETE ON RESTRICT actions.
CREATE DATABASE IF NOT EXISTS dbdemo;
 
USE dbdemo;
 
CREATE TABLE categories(
   cat_id int not null auto_increment primary key,
   cat_name varchar(255) not null,
   cat_description text
) ENGINE=InnoDB;
 
CREATE TABLE products(
   prd_id int not null auto_increment primary key,
   prd_name varchar(355) not null,
   prd_price decimal,
   cat_id int not null,
   FOREIGN KEY fk_cat(cat_id)
   REFERENCES categories(cat_id)
   ON UPDATE CASCADE
   ON DELETE RESTRICT
)ENGINE=InnoDB;
 
MySQL add foreign key
 
MySQL add foreign key syntax
 
To add a foreign key to an existing table, you use the ALTER TABLE statement with the foreign key definition syntax above:
 
ALTER table_name
ADD CONSTRAINT constraint_name
FOREIGN KEY foreign_key_name(columns)
REFERENCES parent_table(columns)
ON DELETE action
ON UPDATE action
 
MySQL add foreign key example
 
Now, let’s add a new table named vendors and change the products table to include the vendor id field:
USE dbdemo;
 
CREATE TABLE vendors(
    vdr_id int not null auto_increment primary key,
    vdr_name varchar(255)
)ENGINE=InnoDB;
 
ALTER TABLE products 
ADD COLUMN vdr_id int not null AFTER cat_id;
 
To add a foreign key to the products table, you use the following statement:
 
ALTER TABLE products
ADD FOREIGN KEY fk_vendor(vdr_id)
REFERENCES vendors(vdr_id)
ON DELETE NO ACTION
ON UPDATE CASCADE;
 
Now, the products table has two foreign keys, one refers to the categories table and another refers to the vendors table.
 
MySQL drop foreign key
 
You also use the ALTER TABLE statement to drop foreign key as the following statement:
 
ALTER TABLE table_name 
DROP FOREIGN KEY constraint_name
 
In the statement above:
 
  -  First, you specify the table name from which you want to remove the foreign key.
  -  Second, you put the constraint name after the DROP FOREIGN KEY clause.
 
Notice that constraint_name is the name of the constraint specified when you created or added the foreign key to the table. If you omit it, MySQL generates a constraint name for you.
 
To obtain the generated constraint name of a table, you use the SHOW CREATE TABLE statement as follows:
SHOW CREATE TABLE table_name
 
For example, to see the foreign keys of the products table, you use the following statement:
 
SHOW CREATE TABLE products
 
The following is the output of the statement:
 
CREATE TABLE products (
  prd_id int(11) NOT NULL AUTO_INCREMENT,
  prd_name varchar(355) NOT NULL,
  prd_price decimal(10,0) DEFAULT NULL,
  cat_id int(11) NOT NULL,
  vdr_id int(11) NOT NULL,
  PRIMARY KEY (prd_id),
  KEY fk_cat (cat_id),
  KEY fk_vendor(vdr_id),
 
  CONSTRAINT products_ibfk_2 
  FOREIGN KEY (vdr_id) 
  REFERENCES vendors (vdr_id) 
  ON DELETE NO ACTION 
  ON UPDATE CASCADE,
 
  CONSTRAINT products_ibfk_1 
  FOREIGN KEY (cat_id) 
  REFERENCES categories (cat_id) 
  ON UPDATE CASCADE
) ENGINE=InnoDB;
 
The products table has two foreign key constraints: products_ibfk_1 and products_ibfk_2.
 
You can drop the foreign keys of the products table by using the following statement:
 
ALTER TABLE products 
DROP FOREIGN KEY products_ibfk_1;
 
ALTER TABLE products 
DROP FOREIGN KEY products_ibfk_2;
 
MySQL disable foreign key checks
 
Sometimes, it is very useful to disable foreign key checks e.g., when you load data into the tables that have foreign keys. If you don’t disable foreign key checks, you have to load data into a proper order i.e., you have to load data into parent tables first and then child tables, which can be tedious. However if you disable the foreign key checks, you can load data into any orders.
 
Another example is that, unless you disable the foreign key checks, you cannot drop a table that is referenced by a foreign key constraint. When you drop a table, any constraints that you defined for the table are also removed.
 
To disable foreign key checks, you use the following statement:
 
SET foreign_key_checks = 0
 
And of course, you can enable it by using the statement below:
 
SET foreign_key_checks = 1
 
In this tutorial, we have covered a lot about MySQL foreign key. We also introduced you to some very handy statements that allow you to manage foreign keys effectively in MySQL.