Schema

The Entity-Relationship Data Model

The procedure to design a database starts with an analysis of what information the database must hold and what are the relationships among elements of that information. Often, the structure of the database, called the database schema, is specified in one of several

Equivalent Representations of a Relation

Relations are sets of tuples, not lists of tuples. Therefore the order in which the tuples of a relation are presented is unimportant. For instance, we can list the three tuples of the

From Entity Sets to Relations

We shall first consider entity sets that are not weak. We shall take up the modifications required to accommodate weak entity sets in "Handling Weak Entity Sets". For each

From E/R Relationships to Relations

Relationships in the E/R model are also represented by relations. The relation for a given relationship R has the following attributes:

Handling Weak Entity Sets

When a weak entity set appears in an E/R diagram, we need to do three things in different ways. 1. The relation for the weak entity set W itself must contain not only the attributes of W but also the key attributes of the other entity sets that help form the

An Object-Oriented Approach

Another strategy for converting isa-hierarchies to relations is to enumerate all the possible subtrees of the hierarchy. For each, create one relation that represents entities that have

Rules About Functional Dependencies

In this section, we shall learn how to reason about FD's. That is, suppose we are told of a set of FDs that a relation satisfies. Sometimes, we can deduce that the relation must satisfy

Computing the Closure of Attributes

Before proceeding to other rules, we shall give a general principle from which all rules follow. Assume {A1, A2,. . . ,An} is a set of attributes and S is a set of FD's. The closure of {A1, A2, .

Projecting Functional Dependencies

When we learn design of relation schemas, we shall also have need to answer the following question about FD's. Assume we have a relation R with some FD's F, and we "project" R by removing certain attributes from the schema. Consider S is the relation that results from R if we

Third Normal Form

Sometimes, one encounters a relation schema and its FD's that are not in BCNF but that one doesn't want to decompose further. The following example is typical.

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