A foreign key references is where one field "links" to another table.
The database will maintain referential integrity - this means that there must be a corresponding record in the other table - in this example each track must be on an album that exists in the album table.
The field (or fields) linked to must be unique - usually the other field is the primary key of the other table.
In this example we store details of tracks on albums - we need to use three columns to get a unique key - each album may have more than one disk - each disk will have tracks numbered 1, 2, 3...
|Specific to SQL Server|