An Entity Relationship (ER) Diagram is a kind of workflow diagram, which demonstrates how “entities” such as people, objects or concepts communicate with each other in a structure. ERDs are mostly utilized in order to design or debug relational databases in the areas of software development departments, information technology departments, for storing and learning in the future and R&D departments. ERDs are also known as ER Models, and it uses certain distinct set of symbols like rectangles, diamonds, ovals and lines for connecting all the symbols and illustrate the connection of entities, relationships and its attributes. The ERDs are like a copy of the grammar in the English language where entities are like nouns and the relationships are like verbs.
ERDs are very similar to the Data Structure Diagrams or DSDs that focuses on the connection of an element within the entity, rather than that of relating entities among themselves. In simple words, relationship between elements inside an entity is more focused on, instead of focusing on the relationships of the entities with themselves. ERDs are also frequently utilized in along with data flow diagrams (DFDs), in order to draw the flow of information needed for the process or system to function.
ERDs consist mainly of four components- Entity, Relationship, Attribute and Cardinality.
Entity: is anything that is definable such as people, a concept, an event or series of event, or an object- anything that can have a stored data regarding it. Examples are: students, teachers, vehicle or any artifacts. Entity is represented as a rectangle.
What is Entity Set? It is similar to entity type- but defined at a particular point in time. For example- cars registered in the month of June or customers purchased in the previous quarter etc.What is Entity type? A collection of things which can be defined, for example customers or cars, although the entity will be for the specific customer or car.
Entities are categorized in 3 types- Strong, Weak or Associate. Strong Entity can define with its own attributes, whereas the weak cannot, but an associate entity links entities in an entity set.
Relationship: is how an entity react with each other or connect with each other. For example- A branded car may be registered in this month. The 2 entities here are car and month, and the relationship is the act of registering, linking the two entities. Relationships are represented as a diamond.
Attribute: is defined as the property or characteristic of an entity. It is represented as a circle.
The 3 categories of the attributes are Simple, Composite and Derived.
Cardinality: It is defined as the numerical attributes of the relationship among the entities- or sets. The types of cardinality are- One-to-One, One-to-Many and Many-to-Many.