ERM

THE ENTITY RELATIONSHIP MODEL (ERM)
ENTITIES
The entity is an object of interest to the end user. entity correspond to the table – not to a row- in the relational environment.
ATTRIBUTES
Attributes are characteristics of entities .the STUDENT entity includes, among many others, the attributes NAME, FNAME, and INITIAL.
ER can be represented by:
- The Chen notation favors conceptual modeling.
- The Crow s Foot notation favors a more implementation-
oriented approach.
- The UML notation can be used for both conceptual and
implementation modeling.
REQUIRED ATTRIBUTES
Is an attribute that must have a value; in other words it cannot be left empty. This attributes represented by a boldface in the Crow s Foot notation.
OPTIONAL ATTRIBUTES
Is an attribute that does not require a value; therefore , it can be left empty and those attributes are not presented in boldface in the entity in the Crow s Foot notation as following figure.



DOMAINS
Domain is the set of possible values for a given attribute For example, the domain for grade point average attribute is written (0,4) because the lowest possible GPA value is 0 and the highest possible value is 4.