E R MODEL
It is a logical tool which is used for database scheme design.it 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
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
Post it in detail
ReplyDeleteKeep following for latest updates..
Delete