E r diagram
- 1. E-R DIAGRAM What is an ER diagram? • An Entity Relationship Diagram (ERD) is a visual representation of different entities within a system and how they relate to each other. Ex: The elements writer, novel, and a consumer may be described using ER diagrams the following way: ER diagram with basic objects • They are also known as ERD’s or ER models.
- 2. ER diagrams in software engineering • Entity relationship diagrams are used in software engineering during the planning stages of the software project. They help to identify different system elements and their relationships with each other. It is often used as the basis for data flow diagrams or DFD’s as they are commonly known. Ex. An inventory software used in a retail shop will have a database that monitors elements such as purchases, item, item type, item source and item price. Rendering this information through an ER diagram would be something like this: ER diagram example with entity having attributes
- 3. ER Diagram Symbols and Notations Elements in ER diagrams There are three basic elements in an ER Diagram: entity, attribute, relationship. There are more elements which are based on the main elements. They are weak entity, multi valued attribute, derived attribute, weak relationship, and recursive relationship. Cardinality and ordinality are two other notations used in ER diagrams to further define relationships.
- 4. Entity An entity can be a person, place, event, or object that is relevant to a given system. For example, a school system may include students, teachers, major courses, subjects, fees, and other items. Entities are represented in ER diagrams by a rectangle and named using singular nouns. Weak Entity • A weak entity is an entity that depends on the existence of another entity. In more technical terms it can be defined as an entity that cannot be identified by its own attributes. It uses a foreign key combined with its attributed to form the primary key. An entity like order item is a good example for this. The order item will be meaningless without an order so it depends on the existence of the order. Weak Entity Example in ER diagrams
- 5. Attribute An attribute is a property, trait, or characteristic of an entity, relationship, or another attribute. Ex: The attribute Inventory Item Name is an attribute of the entity Inventory Item. An entity can have as many attributes as necessary. Meanwhile, attributes can also have their own specific attributes. Ex: The attribute “customer address” can have the attributes number, street, city, and state. These are called composite attributes. Note that some top level ER diagrams do not show attributes for the sake of simplicity. In those that do, however, attributes are represented by oval shapes. Attributes in ER diagrams, Note that an attribute can have its own attributes ( composite attribute )