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MySQL Foreign Key
Summary: 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 another field of another table. A foreign key places constraints on data in the related tables, which enables MySQL to maintain referential integrity.
Let’s take a look at the following database diagram in the sample database.
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 the customerNumber
field. The customerNumber
field in the orders
table relates to the customerNumber
primary key field in the customers
table.
The customers
table is called parent table or referenced table, and the orders
table is known as child table or referencing table.
A foreign key can be a column or 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 may refer to 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 the orders
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 tables are the same. The foreign key refers back to the primary key of the table e.g., the following employees
table:
The reportTo
column is a foreign key that refers to the employeeNumber
column which is the primary key of the employees
table to reflect the reporting structure between employees i.e., each employee reports to another employee and an employee can have zero or more direct reports. We have a specific tutorial on the self-join to help you query data against this kind of table.
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 customerNumber
foreign key to perform the 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.
Creating foreign keys for tables
MySQL creating foreign key syntax
The following syntax illustrates how to define a foreign key in a child table in CREATE TABLE statement.
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CONSTRAINT constraint_name
FOREIGN KEY foreign_key_name (columns)
REFERENCES parent_table(columns)
ON DELETE action
ON UPDATE action
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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 refers to primary key columns in the parent table. You can put a foreign key name afterFOREIGN KEY
clause or leave it to let MySQL 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 the child table and parent table specified in theFOREIGN KEY
andREFERENCES
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 theON 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 ask MySQL to delete records in the child table that refers 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 theON DELETE SET NULL
action instead. MySQL will set the foreign key column values in the child table toNULL
when the record in the parent table is deleted, with a condition that the foreign key column in the child table must acceptNULL
values. Notice that if you useON DELETE NO ACTION
orON 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 theON UPDATE
clause to let MySQL reject any updates to the rows in the child table when the rows in the parent table are updated. TheON UPDATE CASCADE
action allows you to perform a cross-table update, and theON UPDATE SET NULL
action resets the values in the rows in the child table toNULL
values when the rows in the parent table are updated. TheON UPDATE NO ACTION
orUPDATE RESTRICT
actions reject any updates.
MySQL creating 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_id
field in the products
table is defined as a foreign key with UPDATE ON CASCADE
and DELETE ON RESTRICT
actions.
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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;
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Adding a foreign key to a table
MySQL adding 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:
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ALTER table_name
ADD CONSTRAINT constraint_name
FOREIGN KEY foreign_key_name(columns)
REFERENCES parent_table(columns)
ON DELETE action
ON UPDATE action;
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MySQL adding foreign key example
Now, let’s add a new table named vendors
and change the products
table to include the vendor id field:
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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;
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To add a foreign key to the products
table, you use the following statement:
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ALTER TABLE products
ADD FOREIGN KEY fk_vendor(vdr_id)
REFERENCES vendors(vdr_id)
ON DELETE NO ACTION
ON UPDATE CASCADE;
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Now, the products
table has two foreign keys, one refers to the categories
table and another refers to the vendors
table.
Dropping MySQL foreign key
You also use the ALTER TABLE
statement to drop foreign key as the following statement:
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ALTER TABLE table_name
DROP FOREIGN KEY constraint_name;
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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:
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SHOW CREATE TABLE table_name;
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For example, to see the foreign keys of the products
table, you use the following statement:
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SHOW CREATE TABLE products;
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The following is the output of the statement:
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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;
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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:
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ALTER TABLE products
DROP FOREIGN KEY products_ibfk_1;
ALTER TABLE products
DROP FOREIGN KEY products_ibfk_2;
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MySQL disabling foreign key checks
Sometimes, it is very useful to disable foreign key checks e.g., when you import data from a CSV file into a table. 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 in 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:
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SET foreign_key_checks = 0;
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And of course, you can enable it using the statement below:
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SET foreign_key_checks = 1;
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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.