THE ENTITY RELATIONSHIP MODEL(ERM)ENTITIES

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.
Attributes may share a domain. For instance, a student address share the same domain of all possible addresses.


Identifiers (Primary Keys)

Identifies, that is, one or more attributes that uniquely identify each entity instance. Such identifiers are mapped to primary keys (PKs) in tables. Identifiers are underlined in the ERD.
For example, a CAR entity may be represented by:
CAR(CAR-VIN,MOD_CODE,CAR_YEAR,CAR_COLOR)

Composite Identifiers

That is, a primary key composed of more than one attribute, for instance.
CLASS(CRS-CODE,CLASS-SECTION,CLASS_TIME,ROOM_CODE, PROF_NUM).

Composite Attributes

Is an attribute that can be further subdivided to yield additional attributes. for example the attribute ADDRESS can be subdivided into street, city, state.