Chapter 5 Relational Database Design by ER- and EER-to-Relational Mapping Hour 1

Chapter 5

Relational Database

Design by ER- and EER-to-Relational Mapping

Hour 1

Presented & Modified by

Mahmoud Rafeek AlfarraLecturer in CST Kan younis

Chapter 5 Outline

Relational Database Design Using ER-to-Relational Mapping

Mapping EER Model Constructs to Relations

Relational DatabaseDesign by ER- and EER-to-Relational Mapping

Design a relational database schema Based on a conceptual schema design

Seven-step algorithm to convert the basic ER model constructs into relations

Additional steps for EER model

Relational Database Design Using ER-to-Relational Mapping

ER-to-Relational Mapping Algorithm

COMPANY database example Assume that the mapping will create tables

with simple single-valued attributes

Step 1: Mapping of Regular Entity Types For each regular entity type, create a relation R

that includes all the simple attributes of E Called entity relations

• Each tuple represents an entity instance

Step 2: Mapping of Weak Entity Types For each weak entity type, create a relation R

and include all simple attributes of the entity type as attributes of R

Include primary key attribute of owner as foreign key attributes of R

Step 3: Mapping of Binary 1:1 Relationship Types For each binary 1:1 relationship type

• Include primary key of one side as foreign key of the other side.

• Include other attributes of the relationship as attributes to the realtion.

Step 4: Mapping of Binary 1:N Relationship Types For each regular binary 1:N relationship type

• Identify relation that represents participating entity type at N-side of relationship type

• Include primary key of other entity type as foreign key in S

• Include simple attributes of 1:N relationship type as attributes of S

Step 5: Mapping of Binary M:N Relationship Types For each binary M:N relationship type

• Create a new relation S • Include primary key of participating entity types as

foreign key attributes in S • Include any simple attributes of M:N relationship

Step 6: Mapping of Multivalued Attributes For each multivalued attribute

• Create a new relation• Primary key of R is the combination of A and K• If the multivalued attribute is composite, include its

simple components

Step 7: Mapping of N-ary Relationship Types For each n-ary relationship type R

• Create a new relation S to represent R• Include primary keys of participating entity types as

foreign keys• Include any simple attributes as attributes

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Chapter 5

Relational Database

Hour 2

Presented & Modified by

Discussion and Summary of Mapping for ER Model Constructs

In a relational schema relationship, types are not represented explicitly Represented by having two attributes A and B:

one a primary key and the other a foreign key

Mapping EER Model Constructs to Relations

Extending ER-to-relational mapping algorithm

Mapping of Specialization or Generalization

Step 8: Options for Mapping Specialization or Generalization (see pages 294-295) Option 8A: Multiple relations—superclass

and subclasses• For any specialization (total or partial, disjoint or

overlapping)

Option 8B: Multiple relations—subclass relations only

• Subclasses are total• Specialization has disjointedness constraint

Mapping of Specialization or Generalization

Option 8C: Single relation with one type attribute

• Type or discriminating attribute indicates subclass of tuple

• Subclasses are disjoint• Potential for generating many NULL values if many specific

attributes exist in the subclasses

Option 8D: Single relation with multiple type attributes

• Subclasses are overlapping • Will also work for a disjoint specialization

Mapping of Shared Subclasses (Multiple Inheritance)

Apply any of the options discussed in step 8 to a shared subclass

Mapping of Categories (Union Types)

Step 9: Mapping of Union Types (Categories) Defining superclasses have different keys Specify a new key attribute

• Surrogate key

Summary

Map conceptual schema design in the ER model to a relational database schema Algorithm for ER-to-relational mapping Illustrated by examples from the COMPANY

database

Include additional steps in the algorithm for mapping constructs from EER model into relational model

و ذكـ�رو ذكـ�رال. رسـول اللـه صـلى اللـه عليـه وسلم: ق.

من صـلى العشـاء في جماعـة فكأنمـا قـام ـنـصـف ـاللـيـلـ ـومن صـلى الصـبح فـي جـماعـة فكـأنما صـلى الليـل كلهصحيح

Chapter 5

Relational Database

External Practices

Hour 3Presented & Modified by

(a) CUSTOMER entity type with simple attributes

Mapping a regular entity

(b) CUSTOMER relation

(a) CUSTOMER entity type with composite attribute

Mapping a composite attribute

(b) CUSTOMER relation with address detail

Mapping a multivalued attribute

1–to–many relationship between original entity and new relation

Multivalued attribute becomes a separate relation with foreign key

NOTE: the domain constraint for the foreign key should NOT allow null value if DEPENDENT is a weak entity

Foreign key

Composite primary key

Example of mapping a 1:M relationshipRelationship between customers and orders

Note the mandatory one

Mapping the relationship

Again, no null value in the foreign key…this is because of the mandatory minimum cardinality

Foreign key

Example of mapping an M:N relationshipE-R diagram (M:N)

The Supplies relationship will need to become a separate relation

Three resulting relations

New intersection

relationForeign key

Foreign key

Composite primary key

Mapping a binary 1:1 relationshipIn_charge relationship

Resulting relations

Mapping an associative entity with an identifierAssociative entity

Three resulting relations

Mapping a unary 1:N relationship

(a) EMPLOYEE entity with Manages relationship

(b) EMPLOYEE relation with recursive foreign key

Mapping a unary M:N relationship

(a) Bill-of-materials relationships (M:N)

(b) ITEM and COMPONENT relations

Mapping a ternary relationshipTernary relationship with associative entity

Mapping the ternary relationship

Remember that the primary key MUST be

unique

Figure 5-20: Supertype/subtype relationships

Mapping Supertype/subtype relationships to relations

These are implemented as one-to-one relationships

Chapter 5 Relational Database Design by ER- and EER-to-Relational Mapping Hour 1

Documents

Transcript of Chapter 5 Relational Database Design by ER- and EER-to-Relational Mapping Hour 1

Translation of EER model into relational model

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