May 2013 Archive


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

Relation Instances

A relation about movies is not stationary; rather, relations change over time. We expect that these changes include the tuples of the relation, such as insertion of new tuples as movies

From E/R Diagrams to Relational Designs

We now consider the process by which a new database, such as our movie database, is created. We start with a design phase, in which we address and answer questions about what

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:

Combining Relations

Often, the relations that we get from converting entity sets and relationships to relations are not the best possible choice of relations for the given data. One ordinary situation occurs when there

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

Converting Subclass Structures to Relations

When we have an isa-hierarchy of entity sets, we are presented with numerous choices of strategy for conversion to relations. Recall we assume that:

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

Using Null Values to Combine Relations - Comparison of Approaches

There is another way to representing information regarding a hierarchy of entity sets. If we are allowed to use NULL (the null value as in SQL) as a value in tuples, we can handle a hierarchy of entity sets with a single relation. This relation has all the attributes belonging to

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