Database Systems{week 05}

Rensselaer Polytechnic InstituteCSCI-4380 – Database SystemsDavid Goldschmidt, Ph.D.

E/R models (review)

Modeling a database begins by identifying the information to be stored Need to also define how information

elements are related to one another Further, define constraints

on the information,including defining keys,referential integrity, etc.

E/R modeling goals

Fundamental goals of E/R modeling: Capture and document all requirements

and business rules as clearly and exactlyas possible

Satisfy all requirements of the given problem

Produce E/R diagrams that are easily translatable into relational models/implementations

Degree constraints

Each course has many enrolled students Each student enrolls in many classes,

but the maximum for each student is 6

Students Coursesenroll-in

<= 6

Referential integrity

In a referential-integrity constraint, an entity in one set is required to have an entity in another set to which it is related

Each department has exactly one chair Each faculty member can be chair of

at most one department

Faculty Departmentschair-ofC

“exactly one”“at most one”

Subclasses

Much like OOP, we candefine subclasses in anE/R diagram We identify parent/child

entity sets via the is-a relationship

The Students andFaculty relations inherit all attributes of People▪ This includes the key(s) of People

Students Faculty

People

is-a

Specialization via subclasses

Undergrads Graduates

Students

is-a

specialization

generalization

Is this a good design?

A weak entity set is one in which itskey is not guaranteed to be unique

The weak entity set is always linked toone or more (strong) entity sets

Weak entity setskeyB keyA

Weak Entity Set

Strong Entity Set supportC

Weak entities are determined by one or more supporting entities

The key of the supporting entities are appended to the key of the weak entity▪ keyA is not unique, but the combined keyA-keyB

should be unique

Weak entity sets

Strong Entity Set support

keyB keyA

Weak Entity SetC

Dependent names are not guaranteed unique

Names of dependents of a given employee are guaranteed to be unique

Each dependent depends on exactly one employee (required)

Weak entity sets – example #1

Employees depend

empid

name

DependentsC

Course names are not guaranteed to be unique

Names of courses within a given departmentare guaranteed to be unique

Each course is offered in exactly onedepartment (required)

Weak entity sets – example #2

Departments offered

deptid

name

CoursesC

Weak entity sets – example #3

Advisors

Students advisesC

Majors hasC

Faculty serves-asC

Exercises (reminder!)

Go back to the relations we’veworked with in class and createE/R diagrams for each e.g. MusicGroup, Company

Also create a detailed E/R diagram for the celebrities relations (and relationships!) see next slide....

Exercises (reminder!)