About Foreign Key Constraints

A foreign key constraint works in conjunction with primary key or unique constraints to enforce referential integrity among specified tables. For example, you can place a foreign key constraint on the title_id column in the publishers table to ensure that a value entered in that column matches an existing value in the title_id column of the titles table.

In database diagrams, a foreign key constraint is automatically placed on specified columns when you create a relationship to a table from another table to which a primary key or unique constraint is attached.