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In the context of relational databases, a foreign key is a referential constraint between two tables^[1]. The foreign key identifies a column or a set of columns in one (referencing) table that refers to a column or set of columns in another (referenced) table. The columns in the referenced table must form a primary key or unique key. The values in one row of the referencing columns must occur in a single row in the referenced table. Thus, a row in the referencing table cannot contain values that don't exist in the referenced table. This way references can be made to link information together and it is an essential part of database normalization. Multiple rows in the referencing table may refer to the same row in the referenced table. Image File history File links Circle-question-red. ... A relational database is a database that conforms to the relational model, and refers to a databases data and schema (the databases structure of how that data is arranged). ... An example of a database that has not enforced referential integrity. ... In database design, a primary key is a value that can be used to identify a unique row in a table. ... In database design, a unique key refers to the set of columns which have a unique set of values for each row of the table. ... The ASCII codes for the word Wikipedia represented in binary, the numeral system most commonly used for encoding computer information. ... Database normalization is a design technique for structuring relational database tables. ...

The referencing and referenced table may be the same table, i.e. the foreign key refers back to the same table. Such a foreign key is known in SQL:2003 as self-referencing or recursive foreign key.

A table may have multiple foreign keys, and each foreign key can have a different referenced table. Each foreign key is enforced independently by the database system. Therefore, cascading relationships between tables can be established using foreign keys. A database management system (DBMS) is a computer program (or more typically, a suite of them) designed to manage a database, a large set of structured data, and run operations on the data requested by numerous users. ...

Improper foreign key/primary key relationships or not enforcing those relationships are often the source of many database and data modeling problems.

Defining Foreign Keys

Foreign keys are defined in the ANSI SQL Standard, through a FOREIGN KEY constraint. The syntax to add such a constraint to an existing table is defined in SQL:2003 like shown below. Omitting the column list in the REFERENCES clause implies that the foreign key shall reference the primary key of the referenced table.

 ALTER TABLE <table identifier> ADD [ CONSTRAINT <constraint identifier> ] FOREIGN KEY ( <column expression> {, <column expression>}... ) REFERENCES <table identifier> [ ( <column expression> {, <column expression>}... ) ] [ ON UPDATE <referential action> ] [ ON DELETE <referential action> ] 

Likewise, foreign keys can be defined as part of the CREATE TABLE SQL statement.

 CREATE TABLE table_name ( id INTEGER PRIMARY KEY, col2 CHARACTER VARYING(20), col3 INTEGER, ... CONSTRAINT col3_fk FOREIGN KEY(col3) REFERENCES other_table(key_col) ON DELETE CASCADE, ... ) 

If the foreign key is a single column only, the column can be marked as such using the following syntax:

 CREATE TABLE table_name ( id INTEGER PRIMARY KEY, col2 CHARACTER VARYING(20), col3 INTEGER FOREIGN KEY REFERENCES other_table(column_name), ... ) 

Referential Actions

Because the DBMS enforces referential constraints, it must ensure data integrity if rows in a referenced table are to be deleted (or updated). If dependent rows in referencing tables still exist, those references have to be considered. SQL:2003 specifies 5 different referential actions that shall take place in such occurrences:

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

CASCADE

All dependent rows are deleted if the referenced row is deleted. Likewise, the values in the foreign key columns are updated with the respective values in the referenced row.

RESTRICT

A row in the referenced table cannot be updated or deleted if dependent rows still exist. In that case, no data change is even attempted.

NO ACTION

The UPDATE or DELETE SQL statement is executed on the referenced table. The DBMS verifies at the end of the statement execution if none of the referential relationships is violated. The major difference to RESTRICT is that triggers or the statement semantics itself may give a result in which no foreign key relationships is violated. Then, the statement can be executed successfully. An UPDATE statement in SQL changes data in one or more records in a relational database management system. ... A DELETE statement in SQL removes records in a relational database management system. ... A database trigger is procedural code that is automatically executed in response to certain events on a particular table in a database. ...

SET NULL

The foreign key values in the referencing row are set to NULL when the referenced row is updated or deleted. This is only possible if the respective columns in the referencing table are nullable. Due to the semantics of NULL, a referencing row with NULLs in the foreign key columns does not require a referenced row. A foreign key with a cascade delete means that if a record in the parent table is deleted, then the corresponding records in the child table with automatically be deleted. This is called a cascade delete. Columns in Relational database management systems (RDBMS) can optionally store NULL values. ...

SET DEFAULT

Similarily to SET NULL, the foreign key values in the referencing row are set to the column default when the referenced row is updated or deleted.

Example 1

As a first example to illustrate foreign keys, suppose an accounts database has a table with invoices and each invoice is associated with a particular supplier. Supplier details (such as address or phone number) are kept in a separate table; each supplier is given a 'supplier number' to identify them. Each invoice record has an attribute containing the supplier number for that invoice. Then, the 'supplier number' is the primary key in the Supplier table. The foreign key in the Invoices table points to that primary key. The relational schema is the following. Keys are marked in bold.

 Supplier ( SupplierNumber, Name, Address, Type ) Invoices ( InvoiceNumber, SupplierNumber, Text ) 

The corresponding DDL is this: DDL may be an abbreviation for: Daniel Day-Lewis, Academy Award winning Irish/English actor Data Definition Language Data Description Language (see description language) Data Design Laboratory Data Design Language Data Distribution List Dedicated Data Link Delegation Of Disclosure Authority Letter Den Danske Landinspektørforening, the Danish Association of Chartered...

 CREATE TABLE Supplier ( SupplierNumber INTEGER NOT NULL, Name VARCHAR(20) NOT NULL, Address VARCHAR(50) NOT NULL, Type VARCHAR(10), CONSTRAINT supplier_pk PRIMARY KEY(SupplierNumber), CONSTRAINT number_value CHECK (SupplierNumber > 0) ) CREATE TABLE Invoices ( InvoiceNumber INTEGER NOT NULL, SupplierNumber INTEGER NOT NULL, Text VARCHAR(4096), CONSTRAINT invoice_pk PRIMARY KEY(InvoiceNumber), CONSTRAINT inumber_value CHECK (InvoiceNumber > 0), CONSTRAINT supplier_fk FOREIGN KEY(SupplierNumber) REFERENCES Supplier(SupplierNumber) ON UPDATE CASCADE ON DELETE RESTRICT ) 

Example 2

A company has several departments, and each employee is a member of one department. This is enforced in the database by a foreign key on the Employee table, which refers to the Department table. Imagine there is a relation for departments. Each department has a department name, and a surrogate key for the department called "DepartmentID". The employee relation would have "DepartmentID" as an attribute, with a foreign key reference to the aforementioned surrogate key of the department relation. The DBMS would then enforce that each employee could not be created without specifiying a valid department row, and department row could not be deleted if there are employees referencing that department. A surrogate key is a unique primary key generated by the relational database management system that is not derived from any data in the database and whose only significance is to act as the primary key. ...

See also

• alternate key
• candidate key
• compound key
• superkey

An alternate key is any candidate key which is not selected to be the primary key. ... In the relational model a candidate key of a relation variable (relvar) is a set of attributes of that relvar such that (1) at all times it holds in the relation assigned to that variable that there are no two distinct tuples with the same values for these attributes and... In database design, a compound key (also called a composite key) is a key that consists of 2 or more attributes. ... A superkey is defined in the relational model as a set of attributes of a relation variable (relvar) for which it holds that in all relations assigned to that variable there are no two distinct tuples (rows) that have the same values for the attributes in this set. ...

References

1. ^ For a simpler visualization, see VisualCase.com