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Chapter 3. Understanding Relationships > Exclusive Identification

Exclusive Identification

One principle of database design that I'd like to re-emphasize is that a primary key should exclusively identify the value of each field within a given record. Each field value for any particular record should be unique in the database, except where it's used as matching data for establishing a relationship. If the primary key doesn't exclusively identify the field, either it's unnecessary or it belongs in another table.

This is a difficult concept to comprehend. Let's look at the final list of tables and fields in Table 3.26 to see how it applies. Consider the table Programs, where ProgramID is the primary key. Each of the nonkey fields—Name, Location, Synopsis, Notes—is uniquely identified by the primary key. GenreID is not unique, but because it is the primary key of the Genres table, you need the field to establish the relationship between the two tables.


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