Google Code University
From Introduction to Databases and MySQL:Such keys make it possible to relate the tables to one another, so we can, for example, find the customer information given a key value in the order table. A foreign key is a key used to link two tables together. Typically you take the primary key field from one table (customer) and insert it into the other table (order) where it becomes a foreign key (it remains a primary key in the original (customer) table). This type of structure is called a relational database: we have multiple tables that connect to one another via keys.It's really jarring to see such wrongness published under the banner of "Google Code University." I've come to terms with the fact that in the minds of most web/MySQL programmers the term "relational database" means graph/network database. But I still don't expect to see it published by the denizens of a top engineering firm that hires only the best and brightest.