Table Relationships - Database Design for Mere Mortals - page 275

List each of the tables across the top of the matrix, and then again down the left-hand side
of the matrix; make certain the table names are in the same order. Figure 10.26 illustrates
how the matrix should appear.
Figure 10.26. Setting up a table matrix to help identify existing relationships
Select atable ontheleftasastarting pointanddetermine whetherithasarelationship with
any of the tables listed across the top, working your way through the matrix as you do so.
(It doesn't matter whether you work your way across the top or down the side. Just make
sure you work consistently, as it will make the task much easier.)
Keep in mind that you're looking for direct relationships only—there must be a specific
connection between tables participating in the relationship. For example, the CLASSES
table has a direct relationship to the STUDENTS table because one or more students can
attend a given class. Conversely, the CLASSES table has an indirect relationship to the
STAFFtable viatheFACULTYtable; itisa faculty member that teaches aclass, notastaff
member. (You don't have to worry about indirect relationships just yet.)
As you work with a pair of tables, ask the participants questions about the records in each
table. Your goal is to determine the relationship between a single record in one table to one
or more records in the other table, and vice versa. (Remember that each record represents
a single instance of the subject represented by the table.) When you get to a point where
you're examining the same table on both sides of the matrix, try to determine the relation-
shipbetween agiven record inthe table tooneormore other records within the table itself.
There are two types of questions you can ask.
1. Associative: This is a simple and straightforward type of question that you can
generically phrase as follows: Can a single record in (name of first table) be asso-