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
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