A database is a structure that contains information about many different categories of information and about the relationships between those categories (Pratt & Adamsk 2010). Database objects are entities that exist within a database to support operations such as storing, retrieving and manipulating data. Organizations use large amounts of data and database management system to store and transform data into information to support making decisions.
A database management system consists of the following three elements: 1. The physical database: the collection of files that contain the data. 2. The database engine: the software that makes it possible to access and modify the contents of the database. 3. The database scheme: the specifications of the logical structure of the data stored in the database. As we know, database is structured collection of data; computer based databases are usually organized into one or more tables. A table stores data in a format similar to a published table and consists of a series of rows and columns. In a database model, each object that we want to track in the database is known as entitiy. For example, in a college database there might be several entities which is known as set of similar objects. Some of the entities in college database are:
Attributes describes one aspect of an entity type. Entity type is described by set of attributes. An entity is a real-world item or concept that exists on its own (Shiflet, 2002). The set of all possible values for an entity is the entity type. Each entity has attributes, or particular properties that describe the entity. For example student Indra Paudel has properties of his
own studentID, StudentName and StudentGrade. Figure 1 E-R Diagram notation for an attribute domain ( StudentGrade ) of an entity type (Student).
Let’s have a close look of each entity and their attributes. Figure 1.1 the attributes of Student entity.
Figure 1.3 the attributes of course entity.
An entity is a distinguishable object in the enterprise. An entity has attributes that describe the properties of the entity. For example, a course is an object in the student information system. The course number, title, credits, and prerequisites are the attributes for the course. All the courses have same type of attributes. A collection of entities of the same attributes is called an entity set. Since each entity is distinct, no two entities can have the same values on the attributes. Each entity class has an attribute or a set of attributes that can be used to uniquely identify the entities. In case there are several keys in the entity class, we can designate one as the primary key. For example, we can designate the course title to be the key, assume that every course has a different title. A composite attribute is an attribute that is composed of two or more sub-attributes. For example, the Student entity class has the address attribute that consists of street, city, state, and zipcode. A multivalued attribute is an attribute that may consist of a set of values. For example, the Course entity class has the prerequisites attribute. A course may have several prerequisites. Therefore, the prerequisites attribute is a multivalued attribute.
A derived attribute is an attribute that can be derived or calculated from the database. A derived attribute should not be stored in the database. For example, we may add an attribute named numOfPrerequisites to the Course entity class. This attribute can be calculated from the prerequisites attribute. Example of business rule:
A conceptual data model identifies the highest level relationships between the different entities, whereas physical data models represent how the model will be built in the database. A physical database model shows all table structure including column name, column data type, column constraints, primary key, foreign key, and relationships between tables.
Pratt, P.J, & Adamski J.J, (2011). Concepts of Database Management. Ohio, OH: CENGAGE Learning. Shiflet, A.b, (2002). Entity Relationship-Model. Retrieved from http://wofford-ecs.org/dataandvisualization/ermodel/material.htm