Plug-In Essay 3
This Plug-In explains how to design database applications. First of all, we have to learn how to describe the purpose of the relational database model in a database management system. There are numerous elements in a business environment that need to store information, and those elements are related to one to another in a variety of ways so a database must contain not the only information but also information about the relationships among the information. The data model is a formal way to express data relationship to a database management system. There are six database model’s basic components. First ones are entities. An entity is a person, place, thing, transaction, or event about which information is stored and not necessarily tangible, such as an appointment to see the doctor is an entity. Second ones are attributes. Attributes are characteristics or properties of an entity. Third ones are entity identifiers. Entity identifier ensures that each entity instance has a unique attribute value that distinguishes it from every other entity instance. Forth ones are constraints. Constraint is a rule to which some elements in a database must adhere. All entities must have a unique identifier that is constraint. Fifth ones are primary keys. Primary key is a field or group of fields that uniquely identifies a given entity in a table and is important to define in order to retrieve every single piece of information put into database. It should contain some value that can never null and never change. The last ones are foreign keys. A foreign key is primary key of one table that appears as an attribute in another table and acts to provide a logical relation ship between the two tables. Foreign keys may be a part of a concatenated primary key or they may not be part of their tables’s primary key at all. The relational database model is that Dr. Codd introduced in a historic...