Keys are crucial to a table structure for the following reasons:
They ensure that each record in a table is precisely identified. As you already know, a table represents a singular collection of similar objects or events. (For example, a CLASSES table represents a collection of classes, not just a single class.) The complete set of records within the table constitutes the collection, and each record represents a unique instance of the table's subject within that collection. You must have some means of accurately identifying each instance, and a key is the device that allows you to do so.
They help establish and enforce various types of integrity. Keys are a major component of table-level integrity and relationship-level integrity. For instance, they enable you to ensure that a table has unique records and that the fields you use to establish a relationship between a pair of tables always contain matching values.
They serve to establish table relationships. As you'll learn in Chapter 10, you'll use keys to establish a relationship between a pair of tables.
Always make certain that you define the appropriate keys for each table. Doing so will help you guarantee that the table structures are sound, that redundant data within each table is minimal, and that the relationships between tables are solid.