ERDs are used to represent logical relationships between entities. An entity is a person, object, place, orders, products, etc. Information systems recognize relationships between entities. For instance a customer entity can reveal several instances of an order, sales representative, location, among others.
Entity relation diagrams are essential in the design of relational databases. Using an entity relationship data model, abstract schemas are designed and denoted with various entities. Likewise, in object-oriented data model, objects are denoted with different classes with a provision that segregate functional primary data from processes that are involved in creation modification of such data. The functional areas are primarily mapped out during the problem definition stage and should be independent of other processes. This is one crucial stage that should not be overlooked by the design team in order to produce a functional database.
ERDs provide an overall view and blueprint of how the system represents physical data structures. In drawing an ERD, the entities identified during the system analysis stage are linked together to show a logical relationship.
There are numerous ways to represent a ERD using CASE tools but the popular method utilized in this design involve the use of rectangular blocks to represent entities and crows foot to represent relationships.
The entity diagram below has been drawn considering the given below.
One to many relationship: The relationship exists when of the second entity occurs for each instance of the first entity. For instance an office manager heads an office.
One to many relationship: This kind of relationship occurs when one occurrence of the first entity can relate to many occurrences of the second entity but each occurrence of the second entity can relate to one occurrence of the first entity. In the case of the diagram,
