Database Design

Principles of database design

Relational Models

Relational databases are designed to provide efficient structures for transaction processing

Minimize data redundancyIdentify stable structuresProvide for flexible queries

Relational Databases

Based on tables which represent things about which we store data Rows represent instances Columns (attributes) are single

valued properties

Connections among tables are defined by common attributes

Data Modeling

Describe the data structures and relationships that are required to support the users business model. The business model is the abstract description of the rules and processes by which the organization creates value.

Considerations in Relational Database Design

Understand the business environment and uses of data

Identify fields needed to produce required information store data in smallest parts avoid calculated fields

Group fields into tablesDetermine each table’s primary key Include a common field in related tablesAvoid redundancies

Conceptual Data Modeling

Understand the structure of data independent of process

Describe the information objects of the organization and the relationships between objects

Develop from knowledge of the organization and users

EntitiesSomething of importance that can be identified in the business environment.

•Similar entities are grouped in entity classes. Example - individual employees can be grouped in an entity class called EMPLOYEE.

•Individual members of an entity class are called entity instances.

Entities

Entity rows cannot be duplicatesEntities must have more than one

instanceEntities must have more than one

attribute

Examples of Entities

Person - customer, Object - item, productEvent - sale, purchaseConcept - course, flight

Identifying Entities

In the business context, entities are usually described by nouns.

The person, or organization using the system is usually not an entity.

Reports, screens and processing steps are not entities.

Entities with only one attribute are usually modeled as attributes of another entity.

Attributes

Entities have attributes or properties that describe the characteristics of the entity.

All entity instances in an entity class have the same attributes. Attribute values will vary.

Attributes can be composites of other attributes.The type of data described by the attribute is the

same for all entity instances. The value can change but must be the same type (numbers, date, etc).

Keys

Single field or combination of several fields to identify records for retrieval and processing

Primary key - unique identifierSecondary key -identifier, but

not necessarily uniqueForeign key - primary key in

another related table

Example of Relational Tables

Examples of Relational Tables

IdentifiersEntity instances have some name or identification string to identify them within the context of the environment. This name or string is an identifier.

Unique identifiers identify a single entity instance.

Non-unique identifiers can identify sets of instances.

A relationship is a connection or association between entity instances in different entity classes.It specifies what row connects with what row in associated tables.

Relationships

Normalization

A process for identifying structural problems with relational databases to reduce processing anomalies.

Theoretically basedBottom-up (starts with attribute

lists)Step by step procedure

Steps in Normalization

Every non-key attribute is fully functionally dependent on the primary key.

No functional dependencies between non-key attributes.