The one thing that primary, unique, and foreign keys all have in common is the fact that each type of key can consist of more than just one column from a given table. In other words, foreign, primary, and unique keys are not restricted to having just one column from a given table – each type of key can cover multiple columns. Of course, the database programmer is the one who will define which columns are covered by a foreign, primary, or unique key. That is one similarity all those keys share, but there are some major differences that exist between primary, unique, and foreign keys. Let’s go over those differences. We also give a thorough explanation of why foreign keys are necessary in some situations.