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It is a logical tool which is used for database scheme does not include implementation details and described by an ER (Entity-Relationship) diagram.

Entity: An entity is a thing that has an independent existence.It is described by its attributes and determined by its instantiations (are particular values for its attributes).

Entity Set: It is a set of entities of the same type,denote by a rectangular box in ER diagram.Entity can be identified by a list of attributes which are placed in ovals.

Relationship: It is an association among several entities.

Relationship Set: It is a set of relationship of the same type.

super key: A superkey is a set of one or more attributes which collectively allow us to identify uniquely an entity in the entity set.

Candidate Key: A superkey for which no subset is a super key is called a candidate key.

Primary Key: Primary key is a candidate key chosen by the DB designer to identify entities in an entity set.

Weak entity set: An entity set that does not posses sufficient attributes to form a primary key is called a weak entity set. Weak entity set is an entity set whose existence is dependent on one or more other strong entity sets.

Strong entity set: An entity set that does have a primary key is called a strong entity set.

Structural Constraints

Degree: Number of participating entity sets.
Cardinality constraints: Partial or total