As we saw in Chapter 1, we need to specify constraints in the description of a database in order to ensure that the instances we might obtain are meaningful. We distinguished relation constraints, database constraints, dynamic relation constraints and dynamic database constraints. In this chapter, we restrict ourselves to “static” relation constraints. Database constraints will not be discussed in detail; however we do urge the reader to convince himself that there exist database constraints which cannot be expressed by constraints on the relations contained in the database (see Exercise 1.8). Dynamic constraints will be discussed in Chapter 8. Section 3.1 is devoted to some general terminology concerning constraints. In Sections 3.2 till 3.6, we discuss some important types of relation constraints.
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