Appendix B. Relationships, Joins, and Nulls
This appendix expands on some of the concepts that you've learned in this book. Specifically you'll learn more about:
Tools in Access to manage table relationships and query joins
Nulls and their uses
Concepts you need when creating and using lookup tables We'll start by discussing relationships and joins.
Relationships and Joins
All of the database schemas you've studied in this book require you to identify and define relationships between tables. You've also seen examples of, and used, joins in queries. In this appendix, we take a closer look at the tools Access provides to manage those database components. We also demonstrate the effect that the type of relationship or join has on record sets using the data.
A record set is a group of records. This can be as basic as the records in a table or the results returned by a query involving one or more tables.
Note that we used two terms, not just one, to refer to the process of establishing relationships between tables: identify and define. Moreover—unlike in some chapters—we specifically avoided the term create in this context. We'll explain why we make those distinctions shortly, but first, let's briefly review relationships as they exist in databases.
As you recall from Chapter 2, "Elements of a Microsoft Access Database," there are three types of relationships in a database: one-to-one, one-to-many, and many-to-many. In each case, the fundamental principle at work is this: two tables, each ...