Attributes

Representing Other Type Constructors

Above and beyond record structures and sets, an ODL class definition could use Bag, List, Array, or Dictionary to construct values. To represent a bag (multiset), in which a single object can be a member of the bag n times, we cannot simply introduce into a relation n the

Representing ODL Relationships

Generally, an ODL class definition will include relationships to other ODL classes. As in the E/R model, we can create for each relationship a new relation that connects the keys of the two related classes. However, in ODL, relationships come in inverse pairs, and we must create

What If There Is No Key

Since keys are optional in ODL, we may face a situation where the attributes available to us cannot serve to represent objects of a class C uniquely. That situation can be a problem if the class C participates in one or more relationships.

Nested Relations

Relations extended by point (1) of The Object-Relational Model are sometimes called "nested relations''. In the nested-relational model, we allow attributes of relations to have a type that is not atomic: particularly, a type can be a relation schema. Thus, there is a convenient,

Object-Oriented Versus Object-Relational

The object-oriented data model, as typified by ODL, and the object-relational model discussed here, are outstandingly similar. Some of the most important points of comparison follow.

Semistructured Data Representation

A database of semistructured data is a collection of nodes. Each node is either a leaf or interior. Leaf nodes have associated data; the type of this data can be any atomic type, such as numbers and strings. Interior nodes have one or more arcs out. Each arc has a label, which shows

Attribute Lists

There is a strong relationship between XML documents and semistructured data. Assume that for some pair of matching tags and in a document we make a node n. Then, if and are matching tags nested directly within the pair and (i.e., there are no

Relational Algebra

This section begins a study of database programming, that is, how the user can ask queries of the database and can change the contents of the database. Our focus is on the relational model, and especially on a notation for describing queries about the content of relations called

An Algebra of Relational Operations

In order to start our study of operations on relations, we shall learn about a special algebra, called relational algebra that comprises some simple but powerful ways to construct new relations from given relations. When the given relations are stored data, then the constructed

Set Operations on Relations

The three most common operations on sets are union, intersection, and difference. We assume the reader is familiar with these operations, which are described as follows on arbitrary sets R and S:

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