Information Modeling and Relational Databases, 2nd Edition

7.4. Other Constraints and Rules

We now have covered most of the main graphical constraints in ORM. This section considers three more graphical constraints: object cardinality, role cardinality, and value-comparison constraints. It then discusses basic textual constraints and derivation rules. More advanced graphical and textual rules are discussed in Chapter 10.

An object cardinality constraint restricts the cardinality (number of members) of the population of an object type for each state of the database. The cardinality constraint “# = 12” on Jury Member in Figure 7.22(a) means there are always exactly 12 jury members. Here “#” denotes cardinality. Equality settings like this are very rare, as most databases start with the empty state, which ...