This is a flowchart represents relationship between two entities.
ER Diagrams are most often used to design or debug relational databases in the fields of software engineering, business information systems, education and research.
Attributes: Common properties of the entities in a entity sets. Attributes are represented by means of ellipses. Every ellipse represents one attribute and is directly connected to its entity.
Attributes
Composite VS Simple Attributes
Single valued VS Multivalued Attributes
Stored VS Derived Attributes
Complex Attributes
Attributes: If the attributes are composite, they are further
divided in a tree like structure. Every node is then connected to
its attribute. That is, composite attributes are represented by
ellipses that are connected with an ellipse.
Relationship – specify the relations among entities from
two or more entity sets
A relationship where two entities are participating is
called a binary relationship. Cardinality is the number of
instance of an entity from a relation that can be
associated with the relation.
One-to-one
One-to-many
many-to-one
many-to-many
Participation Constraints
Basic concepts
Total Participation − Each entity is involved in
the relationship. Total participation is
represented by double lines.
Partial participation − Not all entities are
involved in the relationship. Partial participation
is represented by single lines
Key is an attribute or collection of attributes that uniquely
identifies an entity among entity set.
Keys
Super Key − A set of attributes (one or more) that
collectively identifies an entity in an entity set.
Candidate Key − A minimal super key is called a
candidate key. An entity set may have more than one
candidate key.
Primary Key − A primary key is one of the candidate keys
chosen by the database designer to uniquely identify the
entity set.
No comments:
Post a Comment