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Definition: Entity Relationship(ER) Model
ER Model is graphical representation of relationship between different entities. An entity is an object about which some data is stored in the database. For example in a school database students, professors, non-teaching staffs, courses, classrooms etc. are entities. A relational database contains data (belonging to these entities) and their relationship with each other as shown in the diagram given below.
So basically ER model graphically represents relationship between different entities which have got certain attributes. Generally these relationships are one-to-one, one-to-many and many-to-many.
One to one relationship: One instance of entity A is related to one instance of entity B. For example one HOD belongs to one department.
One-to-many relationship: One instance of entity A is related to more than one instance of entity B. For example one teacher teaches many students.
Many-to-many relationship: Many instance of entity A is related to many instance of entity B. For example many students studies many subjects.