Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. A relational database matches data by using common characteristics found within the data set. The resulting groups of data are organized and are much easier for people to understand. For example, a data set containing all the real-estate transactions in a town can be grouped by the year the transaction occurred; or it can be grouped by the sale price of the transaction; or it can be grouped by the buyer''s last name; and so on.