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CONVERTING ERDS TO RELATIONAL TABLES

Joe Meehean

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THE PLAYERS ERDs

easy to reason about express lots of information in limited space easy to create from business narrative

Relational Tables easy for DBMSs to store data in tables use SQL to ask lots of different questions about

data Need to convert ERDs to Relational tables

using a set of rules and some intuition

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ENTITY TYPE RULE each entity becomes a table primary key of entity is primary key of table attributes become columns

StudentStudent IDLast nameFirst name

Student ID

Last Name

First Name

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1-M RELATIONSHIP RULE primary key of parent becomes a foreign key

in table of child entity child entity is entity near Crow’s Foot symbol

if minimum cardinality on parent side is 1 (required), foreign key cannot accept null value

Teaches

1-M RELATIONSHIP RULE

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FacultyFaculty IDLast nameFirst name

OfferingSection #RoomTime

Section # Room Time <Faculty ID>

Offering

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Enrolls

M-N RELATIONSHIP RULE M-N relationship becomes it own table primary key is combined key formed from

primary keys of participating entitiesOffering

Section #RoomTime

StudentStudent IDLast nameFirst name

Enrolls

<Student ID> <Section #>

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IDENTIFICATION DEPENDENCY RULE add a component to the primary key of the

weak entity primary key =

primary key of weak entity (if any) + primary keys from independent entities

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IDENTIFICATION DEPENDENCY RULE

HasCourse

Number NameCredits

OfferingSection #RoomTime

<Number>

Section # Room Time

Offering

QUESTIONS?

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Has-Empl

HasIn

In

PartsPart#DescriptionQuantity

DistributorCustomer#NameAddress

Motorcycle

ID#FlamesStyle

Has-JobTitle

OrderOrder#Date$Total

SupplierSupplier#NameAddress

EmployeeEmployee#NameYears

HasYears

JobTitlePosition#NameBaseSalary

Supp

lies

In

QUIZ BREAK!!!Order Quantity

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OPTIONAL 1-M RELATIONSHIPS RULE Optional relationship: minimal cardinality of 0

on parent side (1-side) Convert using the 1-M rule

foreign key in child table (M-side) foreign key can be NULL can be problem for queries

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OPTIONAL 1-M RELATIONSHIPS RULE

Teaches12

FacultyFaculty IDLast nameFirst name

OfferingSection #RoomTime

Section # Room Time <Faculty ID>S1 45 11 NULLS2 68 12 F2S3 58 1 F5S4 35 2 F2

Offering

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OPTIONAL 1-M RELATIONSHIPS RULE Optionally, can use Optional 1-M

Relationship Rule relationship becomes its own table primary keys in both entities become foreign

keys primary key from child entity (M-side) becomes

primary key in new table

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OPTIONAL 1-M RELATIONSHIPS RULE

Teaches14

FacultyFaculty IDLast nameFirst name

OfferingSection #RoomTime

<Section #>

<Faculty ID>

S2 F2S3 F5S4 F5

TeachesSection # Room TimeS1 45 11S2 68 12S3 58 1S4 35 2

Offering

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OPTIONAL 1-M RELATIONSHIPS RULE When to used Optional 1-M Relationship Rule

its optional Optional rule makes more tables

more complex more SQL operations (slower)

1-M rule makes NULL foreign keys can be difficult to deal with

3rd option replace optional relationship with required

relationship and default value

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OPTIONAL 1-M RELATIONSHIPS RULE

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FacultyFaculty IDLast nameFirst name

OfferingSection #RoomTime

Section # Room Time <Faculty ID>

S1 45 11 F0*S2 68 12 F2S3 58 1 F5S4 35 2 F2

Offering

* Assume F0 is PK for “Default Faculty”

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GENERALIZATION HIERARCHY RULE Each entity in a generalization hierarchy

becomes a table Includes only attributes in entity

not its ancestors Except it includes ancestors primary key

uses it as its own primary key also a foreign key

Perform cascading deletes if ancestor is deleted so is subtype table entry

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GENERALIZATION HIERARCHY RULE

StudentMajorGrad Date

FacultyDepartmentOffice

College PeopleCollege IDLast nameFirst name

College ID

Last Name

First Name

<College ID>

Depart-ment

OfficeCollege People Faculty

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1-1 RELATIONSHIP RULE Put a foreign key in each table in the

relationship Unless one table will have many NULL

foreign keys Then drop the foreign key in the table where

it will be mostly NULL

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1-1 RELATIONSHIP RULEFaculty

Faculty IDLast nameFirst name

DepartmentDept. IDFunding

Chairs20

Faculty ID

Last Name

First Name

<Dept ID>

Dept ID Fund-ing

<Faculty ID>

Faculty Department

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1-1 RELATIONSHIP RULEFaculty

Faculty IDLast nameFirst name

DepartmentDept. IDFunding

Chairs21

Faculty ID

Last Name

First Name

Dept ID Fund-ing

<Faculty ID>

Faculty Department

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SELF REFERENCING ENTITIES Apply same rules 1-M rule

add a new column with primary key as foreign key

M-N rule add a new table representing the relationship

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SELF REFERENCING ENTITIES

EmployeeEmployee IDLast nameFirst name

Manages

Employee ID

Last Name

First Name

Supervisor<FK Employee ID>

Employee

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PrerequisiteSELF REFERENCING ENTITIES

CourseNumber NameCredits

Number

Name

Credits

CourseCourse<FK Number>

Prereq<FK Number>

Prerequisites

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CONVERTING ERD REVIEW Every entity becomes a table Some relationships become tables Majority of conversion rules dictate where

foreign key goes foreign key links row in table to primary key in

another table M-N: new table with foreign keys from both

entities 1-M: foreign key in M entity optional 1-M: more complex

QUESTIONS?

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QUIZ BREAK!!!

FacultyFaculty IDLast nameFirst name

StudentStudent IDLast nameFirst name

Major Advisor

Convert to tables

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QUIZ BREAK!!! Convert to tables

HourlyHoursRate

SalarySalaryContract Expires

EmployeeEmployee IDLast nameFirst name

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QUIZ BREAK!!! Convert to tables

InBuilding

Building IDNameAddress

RoomRoom #Capacity

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QUIZ BREAK!!! Convert to tables

Married

FacultyFaculty IDFirst NameLast Name