QUESTION#1:When is the concept of a weak entity used in data modelling? Define theterms owner entitytype, weakentity type, identifyingrelationshiptype, andpartial key. ANSWER:Concept of weak entity is used in conceptualphase of data modelling whileModellingentity types who don’thave key attribute oftheirown. Owner entity type:Aswe know that weak entity type is identified by being related to specificentity from another entity type, so the entity type from which weak entity type is related is called ownerentitytype.Owner entity typehas their own key attribute.
Example: The Student’s unique Roll No will identify the students. So, RollNo is set to be the Primary Key of the STUDENT entity, &Hence STUDENT is an owner entity type because ofits key attribute. Weak entity type: Those entities type which don’t have a key attributes of their own. They dependon other entitiesfor their existence. Example: Dependent entity type cannot exist without Employee entity type in company databasemodel. Identifying relationship type: AnIdentifying Relationship is a relationship between a strong and a weak entitytype, where the keyof the strong entity type is required to uniquely identify instances of the weak entity type.Partial key: It is the attribute which uniquelyidentify weak entities that are related to the sameOwner entity.
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 partialkey. Page 1 of 3QUESTION#2a: What isthe differencebetween a specialization hierarchy andaSpecialization lattice?ANSWER: In specialization hierarchy a subclass can be a subclass in only oneClass/subclass relationship, thatis, each subclass has only one parent, whereas, inspecialization lattice a subclass can be a subclass in more than one class/subclassrelationship, that is, eachsubclass has more than one parent. In above figure subclass STUDENT_ASSISTANT has two parents (EMPLOYEEand STUDENT), whereas, FACULTY subclass has onlyone parent, so STUDENT_ASSISTANT is example of specialization lattice andFACULTY is example of specialization hierarchy. QUESTION#2b:What is the difference between specialization and generalization? Why do we not display thisdifference in schema diagrams? ANSWER: Specialization is a process of defining set of subclasses of an entity set. Specialization is defined on the basis of distinct characteristicsof the entities in superclass. Specialization corresponds to a top-down conceptualrefinementprocess during conceptual schema design.
Generalization is a reverse process of abstraction in which we identify the common featuresof entity and generalize them into a single superclass. It is a process of defining generalize entitytype from the given entity type. Generalization correspondsto a bottom-up conceptual refinement processduring conceptual schema design. Page 2 of 3An arrow pointing to thegeneralized superclass represents a generalization, whereas arrows pointing to the specialized subclasses represent aspecialization.
Wewillnot use this notation becausethe decision as to which process isfollowed in a particularsituation isoften subjective. QUESTION#3: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 thisdatabase. Indicatecardinality of arelationship ANSWER: