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CSE 480: Database Systems

Lecture 6: ER to Relational Mapping

Reference:

Read Chapter 9 of the textbook

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The ER diagram for COMPANY database

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Mapping ER diagram into relational schema

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Mapping ER diagram into relational schema

Map everything as separate tables

ProjNumber ProjName

ProjNumber ProjLocation

Query processing becomes expensive

E.g.: Find me the names of all projects located in Michigan

Need to perform many join operations

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Mapping ER diagram into relational schema

Map everything into 1 table

1 1EMPLOYEE DEPARTMENT

MANAGES

WorksFor

1N

Too much redundancy

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Outline

Mapping ER Constructs to Relations– Step 1: Mapping of Regular (Strong) Entity Types

– Step 2: Mapping of Weak Entity Types

– Step 3: Mapping of Binary 1:1 Relation Types

– Step 4: Mapping of Binary 1:N Relationship Types.

– Step 5: Mapping of Binary M:N Relationship Types.

– Step 6: Mapping of Multivalued attributes.

– Step 7: Mapping of N-ary Relationship Types.

The procedure– Avoids generating too many unnecessary tables

– Avoids too much redundancy in tables

– More details in Chapter 10 (normal forms)

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Step 1: Mapping of Regular (Strong) Entity Types– For each strong entity type E, create a relation R

Include all the simple attributes of E as columns in R Include component attributes of a composite attribute as columns in R Ignore the derived attributes Choose one of the key attributes of E as the primary key for R.

– If the chosen key attribute of E is composite, the set of simple attributes that form it will together form the primary key of R.

ER-to-Relational Mapping Algorithm

E

ID B

C D

Entity type, E

ID C D

R

Relation, R

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Example

PRIMARY KEY (Ssn)

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Example

PRIMARY KEY (Dnumber)UNIQUE(Dname)

PRIMARY KEY (Pnumber)UNIQUE(Pname)

secondary keys(can be null)

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ER-to-Relational Mapping Algorithm

Step 2: Mapping of Weak Entity Types– For each weak entity type W with owner entity type O, create a

relation R

Include all simple attributes of W as columns in R Include components of a composite attribute as columns in R Include primary key attribute(s) of the the owner entity type(s) O as

foreign key attributes of R Primary key of R is the combination of the primary key(s) of the

owner(s) and the partial key of the weak entity type W

O W1 N

Id

A

P

QR

S

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ER-to-Relational Mapping Algorithm

Step 2: Mapping of Weak Entity Types

O W1 N

Id

A

P

QR

S

Oid P R

W

S

PRIMARY KEY (Oid, P)W(Oid) REFERENCES O(Id)

ON DELETE CASCADE

Id A

O

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Example

EMPLOYEESsn

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PRIMARY KEY(Essn, Dependent_name)

DEPENDENT(Essn) REFERENCES EMPLOYEE(Ssn)ON DELETE CASCADE

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ER-to-Relational Mapping Algorithm

Step 3: Mapping of Binary 1-to-1 Relationship types

1 1A B

R

1 1A B

R

1 1A

R

Cross-reference approach: 3 Tables

Foreign key approach: 2 Tables

Merged relation approach: 1 Table

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Cross-Reference Approach

Cross-reference approach:

1 1EMPLOYEE DEPARTMENT

MANAGES

StartDate

MANAGES

MANAGES(Dnumber) REFERENCES DEPARTMENT(Dnumber)MANAGES(Mgr_ssn) REFERENCES EMPLOYEE(Ssn)

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Foreign Key Approach

1 1EMPLOYEE DEPARTMENT

MANAGES

StartDate

Foreign key approach:

DEPARTMENT(Mgr_ssn) REFERENCES EMPLOYEE(Ssn)

Mgr_ssn is NOTNULL

Total participation

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Merged Relation Approach

Merged relation approach:

MANAGER

1 1MANAGER DEPARTMENT

MANAGES

StartDate

Total participation

DEPARTMENT_W_MANAGER

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Summary (Binary 1-1 Relationships)

1 1S T

R

1 1S T

R

1 1S T

R

S

SID

R

SIDTIDP

T

TID

Cross-reference:

S

SID

T_R

TIDSIDP

Foreign key:

S_R_T

SIDTIDP

Merged relation:

P

P

P

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ER-to-Relational Mapping Algorithm

Step 4: Mapping of Binary 1:N Relationship Types.

1 NS T

R

P

S

SID

T_R

TIDSIDP

T_R(SID) references S(SID)

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Example

N 1EMPLOYEE DEPARTMENT

WORKS_FOR

SUPERVISES

Supervisor 1

Subordinate N

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– For each binary 1:N relationship type R Identify the relation S that represents the participating entity type at

the N-side of the relationship type. Include as foreign key in S the primary key of the T Include any simple attributes of R as attributes of S.

Summary (Binary 1-N Relationships)

N 1S T

R

S

SIDTIDP

T

TID

P

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ER-to-Relational Mapping Algorithm

Step 5: Mapping of Binary M:N Relationship Types.

M NS T

R

P

S

SID

R

SIDTIDP

T

TID

R(SID) references S(SID)R(TID) references T(TID)Primary key(SID, TID)

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Example

M NEMPLOYEE PROJECT

WORKS_ON

Hours

Primary key(Essn, Pno)

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Summary (Binary M-N Relationships)

– For each binary M:N relationship type R Create a new relation R Include as foreign key attributes in R the primary keys of the

relations that represent the participating entity types; their combination will form the primary key of R.

Include simple attributes of the relationship type R as attributes of the relation R

M NS T

R

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ER-to-Relational Mapping Algorithm

Step 6: Mapping of Multivalued attributes.

– For each multivalued attribute A, create a new relation T. Include attribute corresponding to A Include primary key attribute K of relation S – as a foreign key in T Primary key of T is the combination of A and K

SA

K

S

KB

T

KA

B

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Example

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ER-to-Relational Mapping Algorithm

Step 7: Mapping of N-ary Relationship Types.

– For each n-ary relationship type R, create a new relation R Include primary keys of the relations participating in the relationship

type as foreign key attributes in R Include any simple attributes of the n-ary relationship type R as

attributes of R

S T

R

P

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Ternary relationship types

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Exercise