Constraints

The Modeling of Constraints

We have studied so far how to model a slice of the real world using entity sets and relationships. However, there are a number of other important features of the real world that

Keys in the E/R Model

A key for an entity set E is a set K of one or more attributes such that, given any two separate entities e1 and e2 in E, e1 and e2 cannot have the same values for each of the

Single-Value Constraints

Sometimes, an important property of a database design is that there is at most one value playing a specific role. For instance, we assume that a movie entity has a unique title, year,

Referential Integrity

While single-value constraints show that at most one value exists in a given role, a referential integrity constraint asserts that exactly one value exists in that role. We could see

Other Kinds of Constraints

As already mentioned at the beginning of this section, there are other kinds of constraints one could wish to enforce in a database. We shall only touch briefly on these here, with the

Functional Dependencies

"From E/R Diagrams to Relational Designs" and "Converting Subclass Structures to Relations" showed us how to convert E/R designs into relational schemas. It is also possible for database designers to produce relational schemas directly from application requirements,

Modification of Constraints

It is possible to add, modify, or delete constraints at any time. The way to express such modifications depends on whether the constraint involved is associated with an attribute, a table, or a database schema.

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