Database ERM and ERD

ENTITY RELATIONSHIP MODLE: (ERM)

ERM uses the ER diagram to represent the conceptual schema(synopsis/outline/diagram)

 which is independent of DBMS. it includes:à

ENTITY TYPE:

Entity s an instance of an entity type that s uniquely identifiable.

S an obj or concept having independent existance / that thing abt which org wana

record data…..existance is of two kinds:

Physical e,g students, teachers

Conceptual e,g sale, examination

TYPES OF ENTITIES:

Æ      Weak Entities:  whose existance depend on strong entity e,g subject

selected. Represented by double rect

Æ      Strong Entities: who has it’s own existance e,g teacher. Represented by

single rect

ATTRIBUTES :

Property of an entity or a relation ship type. represented by oval.

e,g student has name , rno.

Types of attribute:

  1. Simple att: an att composed of single component.. e,g sales , gender(either male /
  2. femal)
  3. composite att: an att composed of multiple components.
  4. single valued att: an att that holds single value for an entity e,g rno
  5. multi valued att: an att that holds multiple value for an entity e,g a person has two telephone #
  6. derived att: an att that represents a value that s derivable from the value of a related att or set of att , not necessarily in the same entity. e,g we can derive age from DOB. So no need to put age because it can be derived any time. e,g age

       KEYS

             all 6 keys . chap 3…

RELATIONSHIP TYPES: any meaningful association among entity types e,g

teaching , paying fees.

Degree of relationship: the number of participating entities in a relation.

uniary (student), binary(student ,teachers), ternary(teachers,student, subject),

Quaternary

Recursive Relationship: a relationship where the same entity participate more than

once in different roles..                                     

ATTRIBUTES ON REALATIONSHIPS: e,g teachers (entity)

checks(relationship) the papers(entity)

But we can put the checking date  which s an att on relationships…

PARTICIPATING CONSTRAINTS:

Employee————completes————–courses—-course ID

                                                                                 –

                                                                              

determies whether the existance of am entity depends upon it’s being related to

another enityt through relation

there could be two problems :

ambigous:……….. FAN trap

missing link: …………chams trap

FAN TRAP: Exist when a model represents a relationship b/w entity types , but the pathway b/w certain entity accurreness is ambiguous(missing link or path doesn't exist)…

                CHASM TRAP:

 

 

                NOTATION                                          MEANING

                                                                                Strong Entity Type

                                                                                Weak Entity Type

                                                                                Weak Relationship

                                                                                Attribute

                                                                                Primary Key Attribute

                                                                                Multi Valued Attribute

                                                                                Derived Attribute

                                                                                Composite Attribute