SQL: Visual QuickStart Guide

Foreign Keys

Information about different entity types is stored in different tables, so you must have a way to navigate between tables. The relational model provides a mechanism called a foreign key to associate two tables. A foreign key has these characteristics:

• It’s a column (or group of columns) in a table whose values relate to, or reference, values in some other table.

• It ensures that rows in one table have corresponding rows in another table.

• The table that contains the foreign key is the referencing or child table. The other table is the referenced or parent table.

• A foreign key establishes a direct relationship to the parent table’s primary key (or any candidate key), so foreign-key values are restricted to parent-key values that already ...