An Entity Relationship Diagram (ERD) is a type of flow chart which shows the relationship of entity sets stored in database. With the help of ER diagram we can represent any object like entities, attributes of an entity, relationship sets, and attributes of relationship sets etc. An Entity Relationship Diagrams are most often used to design or debug relational databases in any field.
ER diagram used to illustrate logical structure of database.
It visualizes how an entity of the system relates with each other. Visualization can be represented with the help of following terms
2] Relationship and Cardinality
A distinguished object can be shown as an entity. I other word, definable object, things, concept or event that can have data stored about it, is called as entity.
Entity is a noun.
Example: – Customer, product etc.
It is represented by rectangle.
2] Relationship and Cardinality:-
Relationship refers the association between two or more than two entities. It helps to identity relations between entities.
It works like verb.
It is represented by diamond.
Example:- one customer can buy many products
Relationship categories into two types:-
A] Strong Relationship
B] Weak Relationship
Cardinality specifies how many instances of an entity related with other entity. It defines numerical relationship between entities or entity set.
Cardinal relationship divides into three main types:-
A] One to One
B] One to Many or Many to One
C] Many to Many
Attributes are associated with properties or characteristics of entity. It is directly connected to the entity.
It is represented by ellipse
Example:- Students has attributes like name, address, contact number etc.
Attributes are divided into three types:-
A] Single value attributes
B] Multi-values attributes
C] Derived attributes
ERD diagram can be used in following way:-
I] Database design
II] Database troubleshooting
III] Business information system
IV] Process re-engineering