Can be person, place, event, object, concept in the real worldCan be physical object or abstractionEx: “John”, “CSE305” GETTING FAMILIAR WITH TERMS
1. ER MODEL : The entity relationship data model on a perception of a real world that
consists of a collection of basic objects called entities and of relationships
among them. This represents the logical structure of an database.
2. ENTITIES : An entity is a thing in the real world that is distinguishable from all
3. ATTRIBUTES : An entity is represented by a s et of attributes. These are descriptive
properties possessed by each member of the entity set.
4. RELATIONSHIP SETS : A relationship set is a set of relationships of the same type.
5. CONSTRAINTS : An E-R enterprise schema may define certain constraints to which the
database must conform.
6. ENTITY SETS : A superkey is a set of one or more attributes that taken
collectively,allow us to identify uniquely an entity in an entity set.
7. N-ARY RELATIONSHIPS : Generally the relationships are binary, sometimes the database has more
than one entity, so they have tertiary or n-ary relationships. This particular
model is a tertiary relationship (three entity sets).
8. MAPPING CARDINALITIES :
ONE-TO-ONE : Every entity in each of the two
entity set are at most related to one entity each.
ONE-TO-MANY: An entity in customer is associated
with a number of entities in order.But Any entity in order can have at most one
entity in customer.
MANY-TO-ONE: An entity in set A is associated
with at most one entity in B. But Any entity
in B can be associated with a number of entities in A.
MANY-TO-MANY: An entity in product can be
associated with a number of entities in order and vice versa is also true.
9. AN E-R DIAGRAM COMPOSES OF
THESE MAJOR COMPONENTS :
RECTANGLES – Entity sets (
CUSTOMER, PRODUCT, ORDER)