entity relationship diagram or re diagram in software engineering - csmates.com

Entity-Relationship Diagrams or er diagram

The Entity-Relationship Diagram (ERD) is the means of providing a data model of the system in structured analysis.

The primary purpose of the ERD is to represent data objects and their relationships. An ERD depicts data at rest. It is used to model data stores in a DFD, independent of processing performed with those data stores.

The main components of an ERD are:
  • Entity
  • Attribute
  • Relationship
An entity is an object or concept about which you want to store information. Attributes are the properties or characteristics of an entity.

An entity is used to represent a set of persons, places, or things all of which have a common name, a common set of properties or attributes. A relationship shows how entities interact and work together. A data attribute helps define entities.

Relationship shows connections among the system's entities. There are 3 major types of relationship used in ERDs.
  • One-to-one relationship.
  • One-to-many relationship
  • Many-to-many relationship
The ERD was originally proposed by Peter Chen for the design of relational database systems, and has been extended for general use by others.

Data objects are represented by a labelled rectangle. Relationships are indicated with a labelled line connecting objects. In some variations of the ERD, the connecting line contains a diamond that is labelled with the relationship.

Steps for Developing an ERD.

Steps in preparing an ERD

  • Identify Entities. Involves identifying and labelled those entities that comprise the system under development. 
  • Indicate Relationship Between Entities. They are connected by lines.
  • Define Keys for Each Entity. Keys are data elements that uniquely identify each entity.
  • Define and Data Elements for Each Entity. Data elements , also called data attributes, represent the data that defines the entities.
  • Normalize the Data Model. Normalization creates a data model that is flexible and stable, with limited data redundancy.
Example of ERD
The relationship between objects in a Book Ordering System.
ERD in software engineering

Connections between data objects and relationships are often labelled using symbols that indicate cardinality ( 1:1=One-to-one, 1:N=One-to-Many, M:N=Many-to-Many ) and modality ( 0=Optional, 1-Mandatory Relationship ).

In most of the cases, the ERD supplies one piece of the analysis model, but it may also be used for detailed database design.

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