When is the concept of a weak entity used in data modelling? Define the
terms owner entity
entity type, identifying
Concept of weak entity is used in conceptual
phase of data modelling while
entity types who don’t
have key attribute of
Owner entity type:
we know that weak entity type is identified by being related to specific
entity from another entity type, so the entity type from which weak entity type is related is called owner
Owner entity type
has their own key attribute.
Example: The Student’s unique Roll No will identify the students. So, Roll
No is set to be the Primary Key of the STUDENT entity, &
Hence STUDENT is an owner entity type because of
its key attribute.
Weak entity type:
Those entities type which don’t have a key attributes of their own. They depend
on other entities
for their existence.
Example: Dependent entity type cannot exist without Employee entity type in company database
Identifying relationship type:
Identifying Relationship is a relationship between a strong and a weak entity
type, where the key
of the strong entity type is required to uniquely identify instances of the weak entity type.
It is the attribute which uniquely
identify weak entities that are related to the same
Example: if we assume that no two dependents of the same employee ever have the same first name,
the attribute Name of DEPENDENT is the partial
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between a specialization hierarchy and
In specialization hierarchy
can be a subclass in only one
Class/subclass relationship, that
is, each subclass has only one parent, whereas, in
specialization lattice a subclass can be a subclass in more than one class/subclass
relationship, that is, each
subclass has more than one parent.
In above figure subclass STUDENT_ASSISTANT has two parents (EMPLOYEE
and STUDENT), whereas, FACULTY subclass has only
one parent, so STUDENT_ASSISTANT is example of specialization lattice and
FACULTY is example of specialization hierarchy.
What is the difference between specialization and generalization? Why do we not display this
difference in schema diagrams?
Specialization is a process of defining set of subclasses of an entity set. Specialization is defined on the basis of distinct characteristics
of the entities in superclass.
Specialization corresponds to a top-down conceptual
process during conceptual schema design.
Generalization is a reverse process of abstraction in which we identify
of entity and generalize
them into a single superclass. It is a process of defining generalize entity
type from the given entity type.
to a bottom-up conceptual refinement process
during conceptual schema design.
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An arrow pointing to the
generalized superclass represents a generalization, whereas arrows pointing to the specialized subclasses represent a
not use this notation because
the decision as to which process is
followed in a particular
Design a Bank
database, including information:
(10) Customer (name, address, phone, and Social
Security number). Account (numbers, type and balances).
Also record the
customer(s) who own an account. Draw the E /R diagram for this
cardinality of a