ASSIGNMENT: Entity Relationship Modelling
Databases, and the database management systems that lord over them, are the core information systems technology. They are used---and will be used---to store corporate data, web pages, on-line movies, work flow information, document databases---absolutely everything that is of interest to business. This is made simpler and possible through a simple approach called entity relationship (ER) model. The ER Model
The ER modelling starts with attributes. Attributes are the characteristics of an entity type that we are interested in. An attribute is a descriptor whose values are associated with individual entities of a specific type. Example the attribute of an employee could be salary. But what then is an entity/entity type? These can better be understood with a simple scenario. Consider the following scenario: Every department within a company is in only one division. Each division has more than one department in it. There are employees or managers in each department The first step is to figure out the items of interest in this situation. It seems here that the situation is concerned with divisions, departments, and employees or managers. These are called entity type. : An entity type is a collection of entities that share a common definition. An entity is a person, place, concept, or thing about which the business needs data. So, Department is the name of one entity type. For instance Marketing division is an instance of the Division entity type. If John was a manager, then John is one instance of the Employee entity type. Instances of entity types are referred to as entities. Put more simply: You can touch an entity but an entity type is simply an idea. Thus, in ER modelling we look for relationships among entity types because it is easier and more concise to speak of relationships among general entity types rather than the touchable entities themselves. From the description there is some sort of...
Please join StudyMode to read the full document