Introduction
In application program development, one of the most important considerations
is the program's ability to accurately represent our perception of the real
world. This requires the program to incorporate a realistic data model. Such a
program is certainly easier to understand than one that performs its task
without using a data model.
In order to develop a data model, we begin with the contention that the real
world consists of entities and relationship.
An entity is defined as a live being, object or abstraction that can be
described in terms of certain characteristic, and is similar to, but distinct
from, entities of the same type.
An entity set is a collection of entities of the same type. For example, the
books in your personal library make up an entity set; this book that you are
reading now is a member of the entity set. You are a student, and therefore, you
are member of the entity students of your college.
Each member of an entity set is different from the other member of the same
set. The difference stem from their individual characteristic, called
attributes. For example, a book is different from all other book because of its
subject matter, author, publisher, design characteristic and unique ISBN book
number. As a member of entity set of students, you are different from the rest
of the student in your college because of your characteristic, such as your
social security number, name, and so on.
In addition to entity set, the real world teems with all kinds of
relationship. A relationship is an association among member of one or more
entity set. A relationship set is a set of similar but distinct relationships of
the same type. For example, class enrollment can be regarded as a relationship
set between the entities sets of students and courses. The fact that you own a
book can be seen as a relationship between you, a member of the entity set of
students, and your book, a member of the entity set of textbooks.
One way you can represent entity set in Visual Basic is by using Classes and
Objects. Fore example, the structure of Students below can be use to represent
the student entity set or any its member in terms of their attributes, such as
Student_ID, FirstName, LastName, MajorCode,
YearLevel and BirthDate.
In Visual Basic, you can represents relationship set by providing the former
entity object with an object property to the latter. For example a typical
Enrollment entity might expose a Student entity. The relationship set enrollment
can be modeled using this structure:Student_ID, CourseNumber, AcademicYear,
Semester and FinalLetterGrade