ER-to-Relational Mapping Principles Specialization/Generalization

Jan. 2012 ACS-3902 Yangjun Chen 1

ER-to-Relational Mapping

• ER-to-Relational Mapping Principles

• Specialization/Generalization

- Superclass/Subclass Relationship

Outline: ER-to-Relational Mapping

Chapter 9 – 3rd ed. (Chap. 7 – 4th, 5th ed.; Chap. 9, 6th ed.)



Sec. 9.1, Sec. 4.1 and 4.2.

General process

1. Create a relation for each strong entity type

2. Create a relation for each weak entity type• include primary key of owner (an FK - foreign key)• owner’s PK + partial key becomes PK

3. For each binary 1:1 relationship choose an entity and include the other’s PK in it as an FK. Include any

attributes of the relationship

4. For each binary 1:n relationship, choose the n-side entity and include an FK w.r.t the other entity. Include any attributes of the relationship



5. For each binary M:N relationship, create a relation for therelationship• include PKs of both participating entities and any attributes

of the relationship• PK is the concatenation of the participating entity PKs

6. For each multivalued attribute create a new relation• include the PK attributes of the entity type• PK is the PK of the entity type and the multivalued attribute

7. For each n-ary relationship, create a relation for the relationship• include PKs of all participating entities and any attributes of

the relationship• PK may be the concatenation of the participating entity PKs



1. Create a relation for each strong entity type

• include all simple attributes

• choose a primary key

Suppose we have:

course 1 Noffered in

Section noterm

meeting

course noname

credit hoursdescription

section



We create a relation for Course - four attributes, course_no is the PK.


Section noterm

meeting

course noname


section

CourseCourse_no name credit_hours description



2. Create a relation for each weak entity type• include primary key of owner (an FK)• Owner’s PK + partial key become the PK

Suppose we have:


Section noterm

meeting

course noname


section



We create a relation for Section


Section noterm

meeting

course noname


section

Section

Course_no Section_no Term

•PK is {course_no, section_no}.

•course_no is an FK.

•meeting is not a simple attribute, so it’s not included.



3. For each binary 1:1 relationship choose an entity and include the other’s PK in it as an FK.

department instructorchair1 1

dept_no dname instr_no iname

There are two choices here

• choose department, or

• choose instructor

Which is the better choice?



Department is the better choice since it must participate in the relationship.

department instructorchair1 1


Department

chairdept_no dname

If we choose department then instr_no is included as, of course, an FK. Note that instr_no must have a value.



4. For each binary 1:n relationship, choose the n-side entity and include an FK w.r.t the other entity.

department instructoremploys1 N


We must choose instructor

We end up with:

instructorinstr_no iname dept_no

•PK is instr_no

•dept_no is an FK

Note that Step 1 would lead to the instructor relation - we have now augmented instructor with the dept_no attribute.



5. For each binary M:N relationship, create a relation for the relationship

• include PKs of both participating entities and any attributes of the relationship

• PK is the catenation of the participating entities’ PKs

student courseenrollm ngrade

Enroll

student_no Course_no grade

•PK is {student_no, course_no}

•student_no is a FK

•course_no is a FK

•grade is an attribute of Enroll

course_nostudent_no



6. For each multi-valued attribute create a new relation• include the PK attributes of the entity type• PK is the PK of the entity type and the multi-valued

attribute


Section noterm

meeting

course noname


section

Meeting is a multi-valued attribute



Create a relation for meeting

Section was created because of Step 2 - its PK is {course_no, section_no}

meeting

Meeting

course_no section_no meeting•PK is {course_no, section_no, meeting}.

•Meeting is an all-key relation.



7. For each n-ary relationship, create a relation for the relationship

• include PKs of all participating entities and any attributes of the relationship

• PK may be the catenation of the participating entity PKs (depends on cardinalities)

semester courseoffersm nroom course_nosemester_no

instructor

instr_no

p



We need one relation, offers, with PK of {semester_no, course_no, instr_no}

semester courseoffersm nroom no course_nosemester_no

instructor

instr_no

p

Offers

course_no instr_no semester_no Room_no



student

graduate undergraduate

Return to Entity-Relationship Modeling

Consider Section 4.2 on Specialization and Generalization

• Specialization is the process of defining a set of sub-entities of some entity type. Generalization is the opposite approach/process of determining a supertype based on certain entities having common characteristics.

• e.g. employees may be paid by the hour or a salary (part vs full-time)• e.g. students may be part-time or full-time; graduate or undergraduate

• these are similar to 1:1 relationships, but they always involve entities of one (super)type

• these are ‘is-a’ relationships

d



student

graduate undergraduate

d

The arc implies graduate and undergraduate are subtypes of student

The bubble and the d imply disjoint subtypes(o - overlap subtypes)

A student must be a graduate or undergraduate

• Participation of supertype may be mandatory or optional

• Participation of subtype is always mandatory

• Subtypes may be disjoint or overlapping

• a predicate (on an attribute) determines the subtype: e.g. attribute Student_class

Student_class = ‘graduate’; Student_class = ‘undergraduate’

Student_class

Subtype is determined by the student_class attribute



Mapping to a relational database - Section 9.2.1 (Step 8)

4 choices:

1. Create separate relations for the supertype and each of the subtypes.

2. Create relations for the subtypes only - each contains attributes from the supertype.

3. (disjoint subtypes) Create only one relation - includes all of the attributes for the supertype and all for the subtypes, and one discriminator attribute.

4. (overlapping subtypes) Create only one relation - includes all of the attributes for the supertype and all for the subtypes, and one logical discriminator attribute per subtype.

PK is always the same - determined from the supertype



SECRETARY ENGINEER

d

Example for super- & sub-types: choice 1

TECHNICIAN

name

lnameminitfname

Ssn bDates Address JobType

TypingSpeed

TGradeEngType

fname, minit, lname, ssn, bdate, address, JobType

EMPLOYEE

Essn, TypingSpeed

SECRETARY

Essn, TGrade

TECHNICIAN

Essn, EngType

ENGINEER

EMPLOYEE



CAR TRUCK

d


VehicleId Price LicensePlate

TNoOfPassengersNoOfAxles

VehicleId, LicensePlate, Price, MaxSpeed, NoOfPassenger

CAR

VehicleId, LicensePlate, Price, NoOfAxles, Tonnage

TRUCK

MaxSpeed Tonnage

Vehicle



SECRETARY ENGINEER

d


TECHNICIAN

name

lnameminitfname

Ssn bDates Address JobType

TypingSpeed

TGradeEngType

fname, minit, lname, ssn, bdate, address, JobType, TypingSpeed, Tgrade, EngType

EMPLOYEE

EMPLOYEE



Part

Manufacture_Part Purchased_Part

o


PartNo Description

manufactureDate

Supplier

PartNo, Desription, MFlag, Drawing, ManufactureDate, BatchNo, Pflag, Supplier, ListPrice

Part

BatchNo

DrawingNoListPrice

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