Attributes

Relational Database Systems

Following a famous paper written by Ted Codd in 1970, database systems changed considerably. Codd suggested that database systems should present the user with a view of data arranged as tables called relations.

Elements of the E/R Model

The most common model for abstract representation of the structure of a database is the entity-relationship model (or E/R model). In the E/R model, the structure of data is represented graphically, as an entity-relationship diagram," using three principal element

Attributes on Relationships

Occasionally it is convenient, or even necessary, to connect attributes with a relationship, rather than with any one of the entity sets that the relationship connects. For example, consider the relationship of Multiway Relationships figure, which represents contracts

Subclasses in the E/R Model

Often, an entity set includes certain entities that have special attributes not connected with all members of the set. If so, we find it useful to describe certain special-case entity sets, or subclasses, each with its own special attributes and/or relationships. We join an entity set to

Design Principles

We have yet to learn many of the details of the E/R model, but we have enough to begin study of very important issue of what forms a good design and what should be avoided. In

Picking the Right Kind of Element

Sometimes we have options about the type of design element used to represent a real-world conception. Many of these choices are between using attributes and using entity

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

Representing Keys in the E/R Model

In our E/R diagram notation, we underline the attributes belonging to a key for an entity set. For instance, the following figure reproduces our E/R diagram for movies, stars, and studios

Requirements for Weak Entity Sets

We cannot get key attributes for a weak entity set indiscriminately. Rather, if E is a weak entity set then its key comprises:

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