A Sample Database Application
In this section we describe a sample database application, called COMPANY, which serves to illustrate the basic ER model concepts and their use in schema design. We list the data requirements for the database here, and then create its conceptual schema step-by-step as we introduce the modeling concepts of the ER model. The COMPANY database keeps track of a company’s employees, departments, and projects. Suppose that after the requirements collection and analysis phase, the database designers provide the following description of the miniworld—the part of the company that will be represented in the database.
- The company is organized into departments. Each department has a unique name, a unique number, and a particular employee who manages the department. We keep track of the start date when that employee began managing the department. A department may have several locations.
- A department controls a number of projects, each of which has a unique name, a unique number, and a single location
- We store each employee’s name, Social Security number,2 address, salary, sex (gender), and birth date. An employee is assigned to one department, but may work on several projects, which are not necessarily controlled by the same department. We keep track of the current number of hours per week that an employee works on each project. We also keep track of the direct supervisor of each employee (who is another employee).
- We want to keep track of the dependents of each employee for insurance purposes. We keep each dependent’s first name, sex, birth date, and relationship to the employee. Figure shows how the schema for this database application can be displayed by means of the graphical notation known as ER diagrams. This figure will be explained gradually as the ER model concepts are presented. We describe the stepby-step process of deriving this schema from the stated requirements—and explain the ER diagrammatic notation—as we introduce the ER model concepts.
Frequently Asked Questions
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Ans: SQL permits a variety of functions on character strings. Read to know about them. view more..
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Ans: Reason to rename a relation is a case where we wish to compare
tuples in the same relation . view more..
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Ans: The natural join operation operates on two relations and produces a relation
as the result. view more..
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Ans: The company is organized into departments. Each department has a unique name, a unique number, and a particular employee who manages the department. We keep track of the start date when that employee began managing the department. A department may have several locations.
A department controls a number of projects, each of which has a unique name, a unique number, and a single location view more..
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Ans: Entities and Their Attributes. The basic object that the ER model represents is an entity, which is a thing in the real world with an independent existence. An entity may be an object with a physical existence (for example, a particular person, car, house, or employee) view more..
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Ans: A database usually contains groups of entities that are similar. For example, a company employing hundreds of employees may want to store similar information concerning each of the employees. These employee entities share the same attributes, but each entity has its own value(s) for each attribut view more..
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Ans: An entity type DEPARTMENT with attributes Name, Number, Locations, Manager, and Manager_start_date. Locations is the only multivalued attribute. We can specify that both Name and Number are (separate) key attributes because each was specified to be unique view more..
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Ans: There are several implicit relationships among the various entity types. In fact, whenever an attribute of one entity type refers to another entity type, some relationship exists. For example, the attribute Manager of DEPARTMENT refers to an employee who manages the department; the attribute view more..
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Ans: Select clause uses. view more..
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Ans: Entity types that do not have key attributes of their own are called weak entity types. In contrast,regular entity types that do have a key attribute—which include all the examples discussed so far—are called strong entity types view more..
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Ans: If some cardinality ratio or dependency cannot be determined from the requirements, the users must be questioned further to determine these structural constraints view more..
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Ans: The individual entity instances in an entity set and the individual relationship instances in a relationship set. In ER diagrams the emphasis is on representing the schemas rather than the instances. view more..
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Ans: The UML methodology is being used extensively in software design and has many types of diagrams for various software design purposes. We only briefly present the basics of UML class diagrams here, and compare them with ER diagrams view more..
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Ans: we defined the degree of a relationship type as the number of participating entity types and called a relationship type of degree two binary and a relationship type of degree three ternary view more..
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Ans: SQL offers the user some control over the order in which tuples in a relation
are displayed. view more..
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Ans: The rapid advancement of the use of information technology (IT) in industry, government, and academia raises challenging questions and problems regarding the protection and use of personal information. Questions of who has what rights to information about individuals for which purposes become more important as we move toward a world in which it is technically possible to know just about anything about anyone. view more..
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Ans: Sensitivity of data is a measure of the importance assigned to the data by its owner, for the purpose of denoting its need for protection. Some databases contain only sensitive data while other databases may contain no sensitive data at all. view more..
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Ans: Whenever a person or a group of persons needs to access a database system, the individual or group must first apply for a user account. The DBA will then create a new account number and password for the user if there is a legitimate need to access the database. view more..
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