4.7. Summary
In step 4 of the conceptual schema design procedure, we add uniqueness constraints and check the arity of fact types. Uniqueness constraints (UCs) come in two varieties: internal and external. An internal (or intrapredicate) UC applies to one or more roles of the same predicate. It is marked as a bar across the constrained role(s). If the roles are noncontiguous, the bar is divided, with a dotted line connecting its active parts.
Redundancy is repetition of an elementary fact. Since redundancy is not permitted in a conceptual fact table, each row must be unique. This is shown as a UC across the whole predicate, unless a stronger UC exists. If an internal UC spans just some of the roles spanned by another internal UC, the former ...
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