Editing Foreign key (section)

== Example ==
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 name and address) 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 Invoice 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)
  Invoice ('''InvoiceNumber''', Text, ''SupplierNumber'')

The corresponding [[Data Definition Language]] statement is as follows.
<syntaxhighlight lang="sql">
CREATE TABLE Supplier (
  SupplierNumber INTEGER NOT NULL,
  Name           VARCHAR(20) NOT NULL,
  Address        VARCHAR(50) NOT NULL,
  CONSTRAINT supplier_pk PRIMARY KEY(SupplierNumber),
  CONSTRAINT number_value CHECK(SupplierNumber > 0)
)

CREATE TABLE Invoice (
  InvoiceNumber  INTEGER NOT NULL,
  Text           VARCHAR(4096),
  SupplierNumber INTEGER NOT NULL,
  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
)
</syntaxhighlight>