Foreign key

In the context of relational databases, a foreign key is a referential constraint between two tables. 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 referencing table must be the primary key or other candidate key in the referenced table. 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 (except potentially NULL). 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. Most of the time, it reflects the one (master table, or referenced table) to many (child table, or referencing table) relationship.

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.

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 as shown below. Omitting the column list in the REFERENCES clause implies that the foreign key shall reference the primary key of the referenced table.

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

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

Referential Actions
Because the Database Management System 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
 * NULL
 * SET DEFAULT

Referential actions are generally implemented as implied triggers (triggers with system-generated names, often hidden.) As such, they are subject to the same limitations as user-defined triggers, and their order of execution relative to other triggers may need to be considered; in some cases it may become necessary to replace the referential action with its equivalent user-defined trigger to ensure proper execution order, or to work around mutating-table limitations.

CASCADE
Whenever rows in the master (referenced) table are deleted, the respective rows of the child (referencing) table with a matching foreign key column will get deleted as well. This is called a cascade delete.

Example Tables: Customer(customer_id, cname, caddress) and Order(customer_id, products, payment)

Customer is the master table and Order is the child table, where 'customer_id' is the foreign key in Order and represents the customer who placed the order. When a row of Customer is deleted, any Order row matching the deleted Customer's customer_id will also be deleted.

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
When the UPDATE or DELETE SQL statement is executed on the referenced table, the DBMS verifies at the end of the statement execution that none of the referential relationships is violated. This is different from RESTRICT because, instead of assuming at the outset that the operation will violate the constraint, the alteration is actually attempted before the constraint invariants are checked. The triggers or the semantics of the statement itself may yield an end state in which no foreign key relationships are violated at the time the constraint is checked, thus allowing the statement to complete successfully.

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.

SET DEFAULT
Similarly 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 it. 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. Primary keys are marked in bold, and foreign keys are marked in italics.

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

The corresponding Data Definition Language statement is as follows.