Chapter 2 - Data Models

entity occurrence

(See entity instance.)

object-oriented data model (OODM)

A data model whose basic modeling structure is an object.

Conceptual Schema

A representation of the conceptual model, usually expressed graphically. See also conceptual model.

Crow's Foot notation

A representation of the entity relationship diagram that uses a three-pronged symbol to represent the "many" sides of the relationship.

Data Definition Language (DDL)

The language that allows a database administrator to define the database structure, schema, and subschema.

object/relational database management system (O/RDBMS)

A DBMS based on the extended relational model (ERDM). The ERDM, championed by many relational database researchers, constitutes the relational model's response to the OODM. This model includes many of the object-oriented model's best features within an inherently simpler relational database structural environment.

attribute

A characteristic of an entity or object. It has a name and a data type.

class

A collection of like objects with shared structure (attributes) and behavior (methods). It encapsulates an object's data representation and a method's implementation. They are organized in a ______ hierarchy.

relational database management system (RDBMS)

A collection of programs that manages a relational database. The ______ software translates a user's logical requests (queries) into commands that physically locate and retrieve the requested data.

logical independence

A condition that exists when the internal model can be changed without affecting the conceptual model. (The internal model is hardware independent because it is unaffected by the choice of computer on which the software is installed. Therefore, a change in storage devices or even a change in operating systems will not affect the internal model.)

physical independence

A condition that exists when the physical model can be changed without affecting the internal model.

entity relationship (ER) model (ERM)

A data model developed by P. Chen in 1975. It describes relationships (1:1, 1:M, and M:N) among entities at the conceptual level with the help of ER diagrams.

network model

A data model standard created by the CODASYL Data Base Task Group in the late 1960s. It represented data as a collection of record types and relationships as predefined sets with an owner record type and a member record type in a 1:M relationship.

relational diagram

A graphical representation of a relational database's entities, the attributes within those entities, and the relationships among the entities.

Physical Model

A model in which the physical characteristics (location, path, and format) are described for the data. Both hardware- and software-dependent. See also physical design.

software independence

A property of any model or application that does not depend on the software used to implement it.

data model

A representation, usually graphic, of a complex "real-world" data structure. They are used in the database design phase of the Database Life Cycle.

constraint

A restriction placed on data. They are normally expressed in the form of rules. Example: "A student's GPA must be between 0.00 and 4.00." They are important because they help to ensure data integrity.

Logical Design

A stage in the design phase that matches the conceptual design to the requirements of the selected DBMS and is therefore software-dependent. It is used to translate the conceptual design into the internal model for a selected database management system, such as DB2, SQL Server, Oracle, IMS, Informix, Access, or Ingress.

entity instance

A term used in ER modeling to refer to a specific table row. Also known as an entity occurrence.

object

An abstract representation of a real world entity that has a unique identity, embedded properties, and the ability to interact with other _________ and itself.

relationship

An association between entities.

internal schema

Depicts a specific representation of an internal model, using the database constructs supported by the chosen database. (The internal model is the representation of a database as "seen" by the DBMS. In other words, the internal model requires a designer to match the conceptual model's characteristics and constraints to those of the selected implementation model.)

Connectivity

Describes the classification of the relationship between entities. Classifications include 1:1, 1:M, and M:N.

relational model

Developed by E. F. Codd (of IBM) in 1970, it represents a major breakthrough for users and designers because of its conceptual simplicity. The _______________, based on mathematical set theory, represents data as independent relations. Each relation (table) is conceptually represented as a matrix of intersecting rows and columns. The relations are related to each other through the sharing of common entity characteristics (values in columns).

relation

In a relational database model, an entity set. They are implemented as tables. They (tables) are related to each other through the sharing of a common entity characteristic (value in a column).

entity set

In a relational model, refers to a grouping of related entities.

internal model

In database modeling, refers to a level of data abstraction that adapts the conceptual model to a specific DBMS model for implementation.

subschema

In the network model, the portion of the database "seen" by the application programs that produce the desired information from the data in the database.

method

In the object-oriented data model, a named set of instructions to perform an action. They represent real-world actions, and are invoked through messages.

inheritance

In the object-oriented data model, the ability of an object to inherit the data structure and methods of the classes above it in the class hierarchy. See also class hierarchy.

hardware independence

Means that a model does not depend on the hardware used in the implementation of the model. Therefore, changes in the hardware will have no effect on the database design at the conceptual level.

business rule

Narrative descriptions of a policy, procedure, or principle within an organization. Examples: A pilot cannot be on duty for more than 10 hours during a 24-hour period. A professor may teach up to four classes during any one semester.

hierarchical model

No longer a major player in the current database market; important to know, however, because the basic concepts and characteristics form the basis for subsequent database development. This model is based on an "upside-down" tree structure in which each record is called a segment. The top record is the root segment. Each segment has a 1:M relationship to the segment directly below it.

many-to-many (M:N OR *..*) relationship

One of three types of relationships (associations among two or more entities) in which one occurrence of an entity is associated with many occurrences of a related entity and one occurrence of the related entity is associated with many occurrences of the first entity.

one-to-many (1:M or 1..*) relationship

One of three types of relationships (associations among two or more entities) that are used by data models. In this relationship, one entity instance is associated with many instances of the related entity.

one-to-one (1:1 or 1..1) relationship

One of three types of relationships (associations among two or more entities) that are used by data models. In this relationship, one entity instance is associated with only one instance of the related entity.

extended relational data model (ERDM)

Sometimes referred to as the enhanced entity relationship model; the result of adding more semantic constructs (entity supertypes, entity subtypes, and entity clustering) to the original entity relationship (ER) model.

semantic data model

The first of a series of data models that more closely represented the real world, modeling both data and their relationships in a single structure known as an object. The ______, published in 1981, was developed by M. Hammer and D. McLeod.

data management language (DML)

The language (set of commands) that allows an end user to manipulate the data in the database (SELECT, INSERT, UPDATE, DELETE, COMMIT, and ROLLBACK).

class hierarchy

The organization of classes in a hierarchical tree where each "parent" class is a superclass and each "child" class is a subclass. See also inheritance.

External Schema

The specific representation of an external view; the end user's view of the data environment.

table

a (conceptual) matrix composed of intersecting rows (entities) and columns (attributes) that represents an entity set in the relational model. also called a relation

entity relationship diagram (ERD)

a diagram that depicts relationship model's entities, attributes, and relations.

schema

a logical grouping of database objects (tables, indexes, views, queries, etc.) that are related to each other. Usually, a ______ belongs to a single user or application.

object-oriented database management system (OODBMS)

data management software used to manage data found within an object-oriented database model.

entity

something about which someone wants to store data; typically a person, a place, a thing, a concept, or an event.

external model

the application programmer's view of the data environment. given its business-unit focuses, it works with a data subset of the global database schema.

conceptual model

the output of the conceptual design process. It provides a global view of an entire database. describes the main data objects, avoiding details.