Transforming an ER Model into a Relational Schema

Cs263 Lecture 10

Transforming an ER Model into a Relational Schema

There are a series of steps that are recommended for converting an E/ER model into a relational schema.

These steps are not hard and fast rules, but rather a series of pertinent suggestions that the database designer must use wisely.

E/ER to Relational Schema Conversion Steps

Step 1 For each ‘strong’ entity in the E/ER model,

create a base relation with a column for each simple attribute of that entity.

The key attribute for the entity becomes the primary key of the relation.

Example – Converting ‘Strong’ Entities

EmpEmpNo

Salary Comm

Hire DateName

Job

EmpNo Number(4)Name Varchar2(10)Job Varchar2(9)Sal Number(7,2)Comm Number(7,2)HireDate Date

Emp Primary Key

Example – Converting ‘Strong’ Entities

Dept

DeptNo

Location

Name

DeptNo Number(2)Name Varchar2(14)Location Varchar2(13)

Dept Primary Key

E/ER to Relational Schema Conversion Steps

Step 2For each ‘weak’ entity, create a relation consisting of all the simple attributes of that entity and also include columns for the primary keys of those entities on whose existence it is dependent.

Example – Converting ‘Weak’ Entities

Loan

LoanNo Char(8)PaymentNo Number(4)Date DateAmount Number(9,2)

Payment Composite Primary Key

PaymentDate

AmountLoan No Payment No

Discriminator

E/ER to Relational Schema Conversion Steps

Step 3When two entities participate in a one-to-many (1-M) relationship, the relation representing the entity with the M (Many) cardinality must have a foreign key column representing this relationship.

Example – Converting (1-M) Relationships

Dept

D#

EmpWorks in

Assigned E#

EmpNo Number(4)Name Varchar2(10)Job Varchar2(9)Sal Number(7,2)Comm Number(7,2)HireDate DateDeptNo Number(2)

Emp

Foreign Key

As an Employee ‘must’ work in a department, then the DeptNo column in the Emp relation (table) cannot be NULL!

Example – Converting (1-M) Relationships

EmpNo Number(4)Name Varchar2(10)Job Varchar2(9)Sal Number(7,2)Comm Number(7,2)HireDate DateDeptNo Number(2)

Emp

Foreign Key

As an Employee ‘may not’ work in a department, then the DeptNo column in the Emp relation (table) can be NULL!

Dept

D#

EmpWorks in

Assigned E#

E/ER to Relational Schema Conversion Steps

Step 4When two entities participate in a (1-1) relationship, a foreign key column must be included in the relation that represents one of these entities.

Example – Converting (1-1) Relationships

Emp

E#

ID CardBelongs to

Has ID#

EmpNo Number(6)Name Varchar2(10)Address Varchar2(200)DOB DateIDNo Char(10)

Emp

Foreign Key

IDNo Char(10)Issued DateExpires Date EmpNo Number(6)

ID Card

Foreign Key

EITHER ONE IS ACCEPTABLE

E/ER to Relational Schema Conversion Steps

Step 5When two entities participate in a many-to-many (M-M) relationship, then a relation must be created consisting of foreign keys for the two relations that represent the participating entities.

Example – Converting (M-N) Relationships

Person

P#

CarDriven by

Drives C#

PersonNo Number(6)CarNo Char(7)

Person-Car Foreign KeyForeign Key

Person

P#

CarC#

Person-Car

P#,C#

E/ER to Relational Schema Conversion Steps

Step 6Where an entity has a multi-valued attribute, create a relation with a column as a foreign key to the relation that represents the entity and a column for the multi-valued attribute.

LanguageLanguage

Example – Multi-valued Attributes

Programmer

EmpNo

Language

Name

EmpNo Number(2)Name Varchar2(14)

Programmer Primary Key

EmpNo Number(2)Language Varchar2(20)

Prog-Lang Composite Primary Key

E/ER to Relational Schema Conversion Steps

Step 7When more than two entities participate in a relationship, then a relation must be created consisting of foreign keys to those relations representing the entities participating in the relationship.

Example – Converting Ternary Relationships

Match

Match No

PlayerPlayer No

Offence

Offence No

Booking

MatchNo Number(4)PlayerNo Number(6)OffenceNo Number(3)Time Date

BookingComposite Primary Key

E/ER to Relational Schema Conversion Steps

Step 8 – Option AFor sub-typing, create separate relations for each subtype consisting of those attributes which are peculiar to the given subtype, and include a foreign key to the super-type, that will act as the sub-type’s primary key.

Create a separate relation for the super-type!

Example – Converting Sub-types

EmployeeNoNameAddress

Employee

EMPLOYEE Employee No Name Address

SECRETARYTyping Speed

TECHNICIANGrade

ENGINEERType

EmployeeNoTypingSpeed

Secretary

EmployeeNoGrade

Technician

EmployeeNoType

Engineer

E/ER to Relational Schema Conversion Steps

Step 8 – Option BFor sub-typing, create separate relations for each subtype consisting of all the attributes of the super-type, and all those which are peculiar to the given subtype. The primary key of the super-type becomes the primary key of each sub-type.

Do not create a relation for the super-type!

Example – Converting Sub-types

VehicleNoLicenceNoPriceNoOfSeats

Car

VEHICLE Vehicle No Licence No Price

CARNo of Seats

TRUCKNo of AxlesTonnage

Truck VehicleNoLicenceNoPriceNoOfAxlesTonnage