transforming er & eer diagrams into relations (chapter 9)

Database ManagementCOP4540, SCS, FIU

Transforming ER & EER diagrams into Relations

(Chapter 9)


Overview• A relatively straightforward process with a well-

defined set of rules.

• Many CASE Tools can automatically perform many of the conversion steps.– CASE tools often cannot model complex data

relationships.– There are sometimes legitimate alternatives where

you will need to choose a particular solution.– You must be prepared to perform a quality check on

the results obtained with a CASE Tool.


Map Simple Regular Entities• Each regular entity type in an ER diagram is

transformed into a relation.• The name given to the relation is generally the same

as the entity type.• Each simple attribute of the entity type become an

attribute of the relation.• Choose one of the key(s) as primary key of the

relation.• How about composite attributes?• How about multi-valued attributes?


Map Composite Attributes• When a regular entity type has a composite attribute, only

the simple component attributes of the composite attribute are included in the new relation.

COSTOMERID

Name

Address

Street

City

State

Zip

CUSTOMER(ID, Name, Street, City, State, Zip)


Map Multi-valued Attributes• When a regular entity type contains a multivalued attribute, two

new relations (rather than one) are created.

• The first relation contains all of the attributes of the entity type except the multi-valued attribute.

• The second relation contains two sets of attributes.

– The primary key from the first relation, which becomes a foreign key of the second relation.

– Multi-valued attribute itself.

– The primary key of the second of relation is the combination of all attributes.

• The name of the second relation should capture the meaning of the multi-valued attribute.


Example

EMPLOYEESSN

Name

Address

Street

City

State

ZipSkills

EMPLOYEE(SSN, Name, State, City, State, Zip)

EMPLOYEE_SKILL(SSN, Skill)


Map Binary One-Many Relationship• Create a relation for each of the two entity types participating in the relationship.

• Include the primary key attribute (or attributes) of the entity on the one-side of the relationship as a foreign key in the relation that is on the many-side of the relationship ( a mnemonic you can use to remember this rule is this: The primary key migrates to many side).

STUDENT

Name

Major_in DEPT

DeptName

SSN N 1

STUDENT(SSN, Name, DeptName) DPET(DeptName)


Map Binary Many-Many Relationships• Suppose that there is a binary relationship (M:N) between two entity types A

and B. For such a relationship, create a new relation C:– Include as foreign key attributes in C the primary key for each of the two

participating entity type.

– These attributes become the primary key of C.

– Any attributes that are associated with the relationship are included with the relation C.

STUDENT COURSEtakingM N

CID

TextSID

NameGrade

STUDENT(SID, NAME) COURSE(CID, Text)

Taking(SID, CID, Grade)


Map Binary One-One Relationship• Binary 1:1 relationship can be viewed as a special case of 1:m relationship. The

process of mapping such a relationship to relations requires to steps:

– first, two relations are created, one for each of the participating entity type.

– Second, the primary key of one of the relations is included as foreign key in the other relation.

EMPLOYEESSN

Name

DEPTManages1 1

DeptName

EMPLOYEE(SSN, Name, DeptName)

DEPT(DeptName)

EMPLOYEE(SSN, Name)

DEPT(DeptName, ManagerSSN)


Map Unary One-Many Relationship• The entity type in the unary relationship is mapped to a relation using the procedure

described before. Then a foreign key attribute is added within the same relation that references the primary key values.

• Note that the foreign key attribute name should reflect the role name on the one-side.

• A recursive foreign key is a foreign key in a relation that references the primary key values of that same relation.

EMPLOYEE Manages

N

1

SSN

Name Supervisee

Supervisor

EMPLOYEE(SSN, Name, SupervisorSSN)


Map Unary Many-Many relationship• With this type of relationship:

– Two relations are created: one to represent the entity type in the relationship and the other an associative relation to represent the M:N relationship itself.

– The primary key of the associative relation consists of two parts: both take their values from the primary key of the other relation.

– Any attribute of the relationship is included in the associative relation.

PART Contains

M

NComponents

Quantity

ItemID

Name

Unit_cost

PART(ID, Name, Unit_Cost)

COMPONENT(ItemID, ComponentID, Quantity)


Map Weak Entities• For each weak entity type, create a new relation and include all of the

simple attributes (or simple components of composite attributes) as attributes of this relation.

• Then, include the primary key of the owner relation as a foreign key attribute in this relation.

• The primary key of the new relation is the combination of this primary key of the owner and the partial key of the weak entity type.

Employees

SSN name addr.

salary

sex

birthdate

name

sex

birthdate relationship

DEPENDANT_OF

DependantsN1


Example Results

Employee( SSN, name, addr, salary sex, birthdate)

Dependants(name, birthdate, sex, relationship, empSSN)

• The relation for the weak entity not only has the attributes of itself, but also has the key attributes of the other entity sets.

• Do not construct a relation for a “double-diamond” relationship.


Map Ternary (n-ary) Relationship• It is recommended that you convert the ternary (n-ary) relationship to a

number of binary relationships, and then transform the diagram into relationships.

SUPPLIER PROJECT

PART

SNameProjName

PartNo

SUPPLY

Quantity

SUPPLIER PROJECT

PART

SNameProjName

PartNo

Quantity

SUPPLYSSSPJ

SP

1 N 1N

1

N


Example Result

SUPPLIER(SName)

PROJECT(ProjName)

PART(PartID)

SUPPLY(SName, ProjName, PartID, Quantity)


Map Supertype/Subtype Relationships

• The relational data model does not yet directly support/subtype relationships.

• There are various strategies that database designer can use to represent these relationships.

• In this lecture, we introduce the most commonly employed strategy.


Mapping Strategy

• Create a separate relation for the supertype and for each of its subtypes.

• Assign to the relation created for the supertype the attributes that are common to all memebers of the supertype, including the primary key.

• Assign to the relation for each subtype the primary key of the supertype, and this primary key is also a foreign key that references the primary key in relation representing the supertype.

• Assign to the relation for each subtype the attributes that are unique to that subtype.


Example

EMPLOYEE

Name Address

Date_hiredSSN

HOURLYEMPLOYEE

SALAREDEMPLOYEE

CONSULTANT

Hourly_rate Annual_salary Stock_option Contract_No. Billing_rate

d

EMPLOYEE( SSN, Name, Address, Date_hired)

HOURLY_EMPLOYEE(SSN, Hourly_Rate) SALARIED_EMPLOYEE(SSN, Annual_salary, Stock_Option)

CONSULTANT(SSN, Contract_No, Billing_rate)


Map Categories

• Problem– Multiple supertypes may have different primary

keys.

• Solution– The concept of surrogate key, a specified new

key attribute for the subtype.


Example

COMPANY PERSON

ACCOUNTHOLDER

BANKHas_AcctM N

U Total_Balance

BName

SSN

CName NameAddress


Example Results

COMPANY(CName, Address, OwnerID)

PERSON(SSN, Name, OwnerID)

ACCOUNTHOLDER(OwnerID)

HASACCOUNT(OwnerID, BName, Total_Balance)

BANK(BName)

transforming er & eer diagrams into relations (chapter 9)

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