Introduction to PostgreSQL Foreign Key Constraint

In PostgreSQL, a foreign key is a column or a group of columns in a table that uniquely identifies a row inanother table.

A foreign key establishes a link between the data in two tables by referencing theprimary key or aunique constraint of the referenced table.

The table containing a foreign key is referred to as the referencing table or child table. Conversely, the table referenced by a foreign key is known as the referenced table or parent table.

The main purpose of foreign keys is to maintain referential integrity in a relational database, ensuring that relationships between the parent and child tables are valid.

For example, a foreign key prevents the insertion of values that do not have corresponding values in the referenced table.

Additionally, a foreign key maintains consistency by automatically updating or deleting related rows in the child table when changes occur in the parent table.

A table can have multiple foreign keys depending on its relationships with other tables.

To define a foreign key, you can use a foreign key constraint.

PostgreSQL foreign key constraint syntax

The following illustrates a foreign key constraint syntax:

[CONSTRAINT fk_name]   FOREIGN KEY(fk_columns)   REFERENCES parent_table(parent_key_columns)   [ON DELETE delete_action]   [ON UPDATE update_action]

In this syntax:

• First, specify the name for the foreign key constraint after theCONSTRAINT keyword. TheCONSTRAINT clause is optional. If you omit it, PostgreSQL will assign an auto-generated name.
• Second, specify one or more foreign key columns in parentheses after theFOREIGN KEY keywords.
• Third, specify the parent table and parent key columns referenced by the foreign key columns in theREFERENCES clause.
• Finally, specify the desired delete and update actions in theON DELETE andON UPDATE clauses.

The delete and update actions determine the behaviors when the primary key in the parent table is deleted and updated.

Since the primary key is rarely updated, theON UPDATE action is infrequently used in practice. We'll focus on theON DELETE action.

PostgreSQL supports the following actions:

• SET NULL
• SET DEFAULT
• RESTRICT
• NO ACTION
• CASCADE

PostgreSQL foreign key constraint examples

The following statements create thecustomers andcontacts tables:

DROP TABLE IF EXISTS customers;DROP TABLE IF EXISTS contacts;CREATE TABLE customers(   customer_id INT GENERATED ALWAYS AS IDENTITY,   customer_name VARCHAR(255)NOT NULL,   PRIMARY KEY(customer_id));CREATE TABLE contacts(   contact_id INT GENERATED ALWAYS AS IDENTITY,   customer_id INT,   contact_name VARCHAR(255)NOT NULL,   phone VARCHAR(15),   email VARCHAR(100),   PRIMARY KEY(contact_id),   CONSTRAINT fk_customer      FOREIGN KEY(customer_id)        REFERENCES customers(customer_id));

In this example, thecustomers table is the parent table and thecontacts table is the child table.

Each customer has zero or many contacts and each contact belongs to zero or one customer.

Thecustomer_id column in thecontacts table is the foreign key column that references the primary key column with the same name in thecustomers table.

The following foreign key constraintfk_customer in thecontacts table defines thecustomer_id as the foreign key:

CONSTRAINT fk_customer   FOREIGN KEY(customer_id)      REFERENCES customers(customer_id)

Because the foreign key constraint does not have theON DELETE andON UPDATE action, they default toNO ACTION.

NO ACTION

The following inserts data into thecustomers andcontacts tables:

INSERT INTO customers(customer_name)VALUES('BlueBird Inc'),      ('Dolphin LLC');INSERT INTO contacts(customer_id, contact_name, phone, email)VALUES(1,'John Doe','(408)-111-1234','john.doe@example.com'),      (1,'Jane Doe','(408)-111-1235','jane.doe@example.com'),      (2,'David Wright','(408)-222-1234','david.wright@example.com');

The following statement deletes the customer id 1 from thecustomers table:

DELETE FROM customersWHERE customer_id= 1;

Because of theON DELETE NO ACTION, PostgreSQL issues a constraint violation because the referencing rows of the customer id 1 still exist in thecontacts table:

ERROR:update or delete on table "customers" violatesforeign key constraint "fk_customer" on table "contacts"DETAIL:Key (customer_id)=(1)is still referencedfrom table "contacts".SQL state:23503

TheRESTRICT action is similar to theNO ACTION. The difference only arises when you define the foreign key constraint asDEFERRABLE with anINITIALLY DEFERRED orINITIALLY IMMEDIATE mode. We'll discuss more on this in the upcoming tutorial.

SET NULL

TheSET NULL automatically setsNULL to the foreign key columns in the referencing rows of the child table when the referenced rows in the parent table are deleted.

First, drop the sample tables and re-create them with the foreign key that uses theSET NULL action in theON DELETE clause:

DROP TABLE IF EXISTS contacts;DROP TABLE IF EXISTS customers;CREATE TABLE customers(   customer_id INT GENERATED ALWAYS AS IDENTITY,   customer_name VARCHAR(255)NOT NULL,   PRIMARY KEY(customer_id));CREATE TABLE contacts(   contact_id INT GENERATED ALWAYS AS IDENTITY,   customer_id INT,   contact_name VARCHAR(255)NOT NULL,   phone VARCHAR(15),   email VARCHAR(100),   PRIMARY KEY(contact_id),   CONSTRAINT fk_customer      FOREIGN KEY(customer_id)  REFERENCES customers(customer_id)  ON DELETE SET NULL);

Second, insert data into thecustomers andcontacts tables:

INSERT INTO customers(customer_name)VALUES('BlueBird Inc'),      ('Dolphin LLC');INSERT INTO contacts(customer_id, contact_name, phone, email)VALUES(1,'John Doe','(408)-111-1234','john.doe@example.com'),      (1,'Jane Doe','(408)-111-1235','jane.doe@example.com'),      (2,'David Wright','(408)-222-1234','david.wright@example.com');

Third, delete the customer with id 1 from thecustomers table:

DELETE FROM customersWHERE customer_id= 1;

Because of theON DELETE SET NULL action, the referencing rows in thecontacts table are set to NULL.

Finally, display the data in thecontacts table:

SELECT * FROM contacts;

Output:

contact_id | customer_id | contact_name |     phone      |          email------------+-------------+--------------+----------------+--------------------------          3 |           2 | David Wright | (408)-222-1234 | david.wright@example.com          1 |        null | John Doe     | (408)-111-1234 | john.doe@example.com          2 |        null | Jane Doe     | (408)-111-1235 | jane.doe@example.com(3 rows)

The output indicates that the values of customer id 1 changed toNULL.

CASCADE

TheON DELETE CASCADE automatically deletes all the referencing rows in the child table when the referenced rows in the parent table are deleted. In practice, theON DELETE CASCADE is the most commonly used option.

The following statements recreate the sample tables with the delete action of thefk_customer changes toCASCADE:

DROP TABLE IF EXISTS contacts;DROP TABLE IF EXISTS customers;CREATE TABLE customers(   customer_id INT GENERATED ALWAYS AS IDENTITY,   customer_name VARCHAR(255)NOT NULL,   PRIMARY KEY(customer_id));CREATE TABLE contacts(   contact_id INT GENERATED ALWAYS AS IDENTITY,   customer_id INT,   contact_name VARCHAR(255)NOT NULL,   phone VARCHAR(15),   email VARCHAR(100),   PRIMARY KEY(contact_id),   CONSTRAINT fk_customer      FOREIGN KEY(customer_id)  REFERENCES customers(customer_id)  ON DELETE CASCADE);INSERT INTO customers(customer_name)VALUES('BlueBird Inc'),      ('Dolphin LLC');INSERT INTO contacts(customer_id, contact_name, phone, email)VALUES(1,'John Doe','(408)-111-1234','john.doe@example.com'),      (1,'Jane Doe','(408)-111-1235','jane.doe@example.com'),      (2,'David Wright','(408)-222-1234','david.wright@example.com');

The following statement deletes the customer id 1:

DELETE FROM customersWHERE customer_id= 1;

Because of theON DELETE CASCADE action, all the referencing rows in thecontacts table are automatically deleted:

SELECT * FROM contacts;

Output:

contact_id | customer_id | contact_name |     phone      |          email------------+-------------+--------------+----------------+--------------------------          3 |           2 | David Wright | (408)-222-1234 | david.wright@example.com(1 row)

SET DEFAULT

TheON DELETE SET DEFAULT sets the default value to the foreign key column of the referencing rows in the child table when the referenced rows from the parent table are deleted.

Add a foreign key constraint to an existing table

To add a foreign key constraint to the existing table, you use the following form of theALTER TABLE statement:

ALTER TABLE child_tableADD CONSTRAINT constraint_nameFOREIGN KEY (fk_columns)REFERENCES parent_table (parent_key_columns);

When adding a foreign key constraint withON DELETE CASCADE option to an existing table, you need to follow these steps:

First, drop existing foreign key constraint:

ALTER TABLE child_tableDROP CONSTRAINT constraint_fkey;

Second, add a new foreign key constraint withON DELETE CASCADE action:

ALTER TABLE child_tableADD CONSTRAINT constraint_fkFOREIGN KEY (fk_columns)REFERENCES parent_table(parent_key_columns)ON DELETE CASCADE;

Summary

• Use foreign keys to ensure the referential integrity and consistency of data between two tables.
• Use theFOREIGN KEY constraint to define a foreign key constraint when creating a table.
• Use theALTER TABLE ... ADD CONSTRAINT ... FOREIGN KEY to add a foreign key constraint to an existing table.

Last updated onJanuary 25, 2024

On this page

• Introduction to PostgreSQL Foreign Key Constraint
• PostgreSQL foreign key constraint syntax
• PostgreSQL foreign key constraint examples
• Add a foreign key constraint to an existing table
• Summary