6.1. Introduction to CSDP Step 6

So far you have learned how to verbalize examples in terms of elementary facts, draw the fact types, mark uniqueness constraints and mandatory roles, specify rules for derived fact types, and use simple and complex reference schemes to identify entities. The next step of the conceptual schema design procedure covers three kinds of constraints: value, set comparison, and subtype. Set-comparison constraints are themselves of three kinds: subset, equality, and exclusion.

This chapter covers step 6 in detail. To clarify the formal concepts underlying the constraints, some basic set theory is first reviewed. Then we consider value constraints (i.e., restrictions on value types). For example, in modeling color monitors we might restrict the values of ColorCode to ‘R’, ‘G’, and ‘B’ (for Red, Green, and Blue). After discussing the related notion of independent objects, we examine the three set-comparison constraints—these declare whether the population of one role sequence must be included in, be equal to, or be mutually exclusive with the population of another.

After that, we examine the notion of subtyping in some depth. Subtyping allows us to declare how types are related (e.g., each manager is an employee) and refines our ability to declare precisely what kinds of objects play what roles. Our treatment of subtyping proceeds from basics through to some reasonably advanced aspects.