Data Analysis for Database Design, 3rd Edition



Connection traps

 ,QWURGXFWLRQ

This chapter examines several pitfalls which the designers and users of a conceptual model

must take care to avoid. In each case the problem arises because of a misinterpretation of

the meaning of certain relationships. The term connection trap will be used to describe any

such interpretation error; the terms fan trap and chasm trap will be applied to particular

cases to indicate the cause of the problem. Any conceptual model will contain potential

connection traps. Many of these may be of no significance to the enterprise; others can be

avoided by restructuring the conceptual model. Either way, their existence must be

recognised so that they can be eliminated or else given a clean bill of health. We will start

with some trivial cases, and progress to those that are rather less obvious.

 0LVLQWHUSUHWDWLRQ

The meaning of any relationship type must be carefully defined, and thoroughly

understood, in order to avoid errors of misinterpretation. For example, the relationship

Tutors in Fig. 11.1 could refer to subject-tutorship, personal-tutorship, or both of these.

Fig. 11.1 What does Tutors mean? Subject-tutorship? Personal-tutorship? Both of these?

An error in the interpretation of the meaning may cause the database to be incapable of

storing certain information (e.g. personal as distinct from subject tutorships), or may

mean that information retrieved from the database is interpreted incorrectly (e.g. a list

of tutors might be interpreted as being a list of personal tutors when it is actually a list

of subject tutors).

Question

1.

The above entity-relationship structure can represent: