A relational database consists of one or more tables containing data, where the data in one table may relate to data in one or more of the other tables. Tables are related by sharing key values between their rows, and these related tables are joined together in SELECT statements to produce relational result sets. A database containing two tables, Company and Employee, for example, may look something like what is shown in Figure 5-1. Figure 5-1: An example of two related tables. For now, don’t worry about the notation being used—that is all explained in Chapter 8—just look at how the data itself is related. The data contained in these two tables may look something like what is shown in Figure 5-2. As you can see from the matching values of CompanyID between the two tables, some employees are related to one company, and some to another. Visualizing these rows as slightly separated into their own related groups, as shown in Figure 5-3, sometimes helps
