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 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
Property of an entity or a relation ship type. represented by oval.
e,g student has name , rno.
Types of attribute:
- Simple att: an att composed of single component.. e,g sales , gender(either male /
- composite att: an att composed of multiple components.
- single valued att: an att that holds single value for an entity e,g rno
- multi valued att: an att that holds multiple value for an entity e,g a person has two telephone #
- 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
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),
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…
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)…
Strong Entity Type
Weak Entity Type
Primary Key Attribute
Multi Valued Attribute