Why are some table structures considered to be bad and others good and how do you recognize the difference between good and bad structures?
From an information management point of view, possibly the most vexing and destructive problems are created through uncontrolled data redundancies. Such redundancies produce update and delete anomalies that create data integrity problems. The loss of data integrity can destroy the usefulness of the data within the database. (If necessary, review Chapter 1, Section 1.4.4, “Data Redundancy”, to make sure that your students understand the terminology and that they appreciate the dangers of data redundancy.)
Table structures are | | | |Also remind your students that some business rules cannot be incorporated in the ERD, regardless of the level of business rule detail or the | |completeness of the normalization process. For example, the business rule that specifies the constraint | |“A pilot may not perform flight duties more than 10 hours per 24-hour period.” | |cannot be modeled in the ERD. However, tools such a Visio do allow you to write “reminders” of such constraints as text. Because such constraints | |cannot be modeled, they must be enforced through the application software. | Answers to Review Questions
1. What is normalization?
Normalization is the process for assigning attributes to entities. Properly executed, the normalization process eliminates uncontrolled data redundancies, thus eliminating the data anomalies and the data integrity problems that are produced by such redundancies.
Normalization does not eliminate data redundancy; instead, it produces the carefully controlled redundancy that lets us properly link database tables.
