3 & 4 1 Chapters 3 and 4 Drawing ERDs October 16, 2006 Week 3.

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Chapters 3 and 4Drawing ERDs

October 16, 2006

Week 3

Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel

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Practice … (correction)

• Each of the MegaCo Corporation’s divisions is composed of many departments. Each of those departments has many employees assigned to it, but each employee works for only one department. Each department is managed by one employee, and each of those managers can manage only one department at a time.

EMPLOYEE

is assigned to

M

1DEPARTMENT

manages

1

1

DIVISION1M

is composed of


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A little from Chapters 3 and 4

• Good design begins by identifying appropriate entities and attributes and the relationships among the entities

• Chapter 3: Section 3.2 Keys (pages 64-69)

• Chapter 4 All


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The Entity Relationship Diagram (ERD)

• ERD represents the conceptual database as viewed by end user

• ERDs depict the ER model’s three main components:

– Entities - anything about which data are to be collected and stored (i.e. people, events)

– Attributes - characteristics of an entity

– Relationships - describe an association among (two or more) entities (1:1, 1:M, M:N)


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Tables and Keys

• Tables are basic building blocks of a relational database

• Keys are central to the use of relational tables


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Tables and Their Characteristics


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Keys

• Consists of one or more attributes that determine other attributes

• Primary key (PK) is an attribute (or a combination of attributes) that uniquely identifies any given row– Composite key - Composed of more than one attribute

• Foreign key (FK) – An attribute whose values match primary key values in

the related table

• Key’s role is based on determination– If you know the value of attribute A, you can look up

(determine) the value of attribute B


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Keys (continued)


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Keys (continued)• Nulls:

– No data entry– Not permitted in primary key– Should be avoided in other attributes– Can represent

• An unknown attribute value• A known, but missing, attribute value• A “not applicable” condition

– Can create problems when functions such as COUNT, AVERAGE, and SUM are used

– Can create logical problems when relational tables are linked


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Keys (continued)


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Entities and Instances

• Each entity ultimately becomes a table in a database implementation

• Each instance of an entity is a record or a row in a table in a database

• Entity name, a noun, is usually written in capital letters

STUDENT


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Attributes

• Attributes ultimately become the fields in a database implementation

• Attributes are represented by ovals and are connected to entity rectangle with a line

• Primary Key is underlined in the ERD


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Types of Attributes

• Key attribute is a unique identifier

• Composite attribute can be subdivided

• Simple attribute cannot be subdivided

• Single-value attribute can have only a single value

• Multivalued attributes can have many values– A student may have more that one phone number; a home-

phone number and a mobile-phone number

• Derived attribute’s value may be calculated (derived) from other attributes– Need not be physically stored in a database


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Multivalued Attributes

Represented by a double line


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Resolving Multivalued Attribute Problems

• Although conceptual model can handle M:N relationships and multivalued attributes, you should not implement them in relational DBMS– Within original entity, create several new

attributes, one for each of the original multivalued attribute’s components

• Can lead to major structural problems in table

– Create new entity composed of original multivalued attribute’s components


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Resolving Multivalued Attribute Problems (continued)


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Derived Attributes (continued)

Represented by a dashed line


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Connectivity and Cardinality

• Connectivity– Used to describe the relationship classification

(based on participation)

• Cardinality – Expresses minimum and maximum number of

entity occurrences associated with one occurrence of related entity

• Established by very concise statements known as business rules


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EMPLOYEE is assigned to PROJECT

M N

Cardinality

Connectivity


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Relationship Participation

• Optional participation– One entity occurrence does not require

corresponding entity occurrence in particular relationship

• Mandatory participation– One entity occurrence requires corresponding

entity occurrence in particular relationship


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Relationship Strength

• Weak (non-identifying) relationships– Exists if PK of related entity does not contain

PK component of parent entity

• Strong (Identifying) Relationships– Exists when PK of related entity contains PK

component of parent entity


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Weak (Non-Identifying) Relationships


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Strong (Identifying) Relationships


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Weak Entities

• Weak entity meets two conditions– Existence-dependent

• Cannot exist without entity with which it has a relationship

– Has primary key that is partially or totally derived from parent entity in relationship

• Database designer usually determines whether an entity can be described as weak based on business rules


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Weak Entities (continued)


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Relationship Degree

• Indicates number of entities or participants associated with a relationship

• Unary relationship– Association is maintained within single entity

• Binary relationship – Two entities are associated

• Ternary relationship – Three entities are associated


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Relationship Degree (continued)


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Composite Entities

• Also known as bridge entities

• Composed of primary keys of each of the entities to be connected

• May also contain additional attributes that play no role in connective process

• Used to resolve M:N relationships


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29Source: http://io.uwinnipeg.ca/~rmcfadye/2914/ERD/images/1entit4.gif

Mandatory participation

1, N, M


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Practice ...

• An airplane has a registration number, type, number of economy class seats, number of business class seats and the year of production


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Practice ...

• Each text book has a unique ISBN (International Standard Book Number), and contains several chapters. Each chapter has a chapter number (unique within a book), the number of pages and the number of references. A chapter covers a single topic, but the same topic may be covered in various books.


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Practice ...

• Sometimes students work in groups. Each group has a unique number and students have their student ids. A student who works in a group has a specific role within that group. The student may have different roles in various groups he/she belongs to.

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3 & 4 1 Chapters 3 and 4 Drawing ERDs October 16, 2006 Week 3.

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