Module 2

Designing a Logical Database Model

Module Overview

• Guidelines for Building a Logical Database Model

• Planning for OLTP Activity

• Evaluating Logical Models

Lesson 1: Guidelines for Building a Logical Database Model

• Guidelines for Transforming a Conceptual Database Design into a Logical Model

• Best Practices for Working with Entities and Attributes

• Considerations for Selecting Primary Keys

• Best Practices for Finalizing the Logical Model

• Discussion: Creating a Simple Database Model

Guidelines for Transforming a Conceptual Database Design into a Logical Model

Use an automated design tool

Design an initial ER diagram

Use independent naming conventions

Validate the ER diagram with the development team

Use portable database types

Best Practices for Working with Entities and Attributes

Identify independent and dependent entities

Distinguish between tables and views

Specify attribute constraints

Identify special security requirements for attributes

Identify candidate keys

Considerations for Selecting Primary Keys

Natural keys have the following advantages:• Enforced compliance • Less joins• Automatic constraints• User verifiable

Surrogate keys have the following advantages:• Smaller size• Better support for primary key

changes• Better availability• Easier joins

Surrogate Keys

Natural Keys

Each database table has a column or a combination of columns containing values that uniquely identify each row in the table

Best Practices for Finalizing the Logical Model

Identify remaining constraints Identify schemas for grouping entities

Normalize all data tables Compare the conceptual and logical models

Discussion: Creating a Simple Database Model

Scenario:You are a database designer for a small school named Elm HighSchool. The school management needs following data with respect to students and teachers:

• Tracking of each student • The courses for which a particular student is enrolled and is/her grades for each term.• Teachers who teach a particular course• Students who learn from particular teachers

In addition, the management needs the classroom allocation orscheduling information.

Questions:• Identify the probable entities and their characteristics.• Discuss the differences and issues between the conceptual model and the logical model of the proposed database.

Lesson 2: Planning for OLTP Activity

• Guidelines for Identifying Functional Dependencies

• Benefits of Performing Data Normalization

• Degrees of Normalization

• Achieving a Normalized Design

• Guidelines for Performing Denormalization

• Discussion: Normalizing to an Absurd Level

Guidelines for Identifying Functional Dependencies

Identify the semantics of attributes

Identify the concepts of a relational schema

SchemaRelation Tuple KeyAttribute

Benefits of Performing Data Normalization

Reduce redundant values in tuples

Reduce or eliminate null values in tuples

Avoid generating spurious tuples

OhioOhio

-.-.-.-.-.

Degrees of Normalization

First normal form (1NF)

• Relations do not contain any repeating groups• Every attribute is atomic

Third normal form (3NF)

• Relation is in the second normal form • Some attributes of the relation are not

transitively dependent on the primary key

Name

Age

ID

Second normal form (2NF)

• Relation is in the first normal form• Attributes of the relation depend on the whole key

NameAgeSkills

NameE-mail

Phone

Achieving a Normalized Design

Ensure dependency of all values on simple and on composite keys

Ensure that the design process is reversible

Avoid update anomalies with normalization

Ensure a high degree of normalization

Use denormalization when:• Pre-aggregating data from other tables• Avoiding multiple or complex joins• Improving performance

Guidelines for Performing Denormalization

Conditions to Denormalize a Design

• Performance with the normalized design is unacceptable• Performance improves if denormalization is applied• Update anomalies caused by denormalization can be predicted

• Simplifying data access• Providing reporting data• Preventing calculations based on values from the same row

Avoid denormalization when:

Discussion: Normalizing to an Absurd Level

Scenario:Normalization is about reducing duplicated data. 1234 Flower Blvd

Normalize into the following fields:AddressNumber | AddressNameID | AddressTypeID1234 412 14

AddressNameID 412 is ‘Flower’ in the AddressName tableAddresssTypeID is ‘Blvd’ in the AddressType table

Questions:• Why would you not normalize to the level of absurdity?• Provide a business scenario where an absurd level of data

normalization could prove useful.

• Guidelines for Analyzing an Existing Logical Model

• Identifying Problems with a Logical Model

• Discussion: Expanding Legacy Projects

Lesson 3: Evaluating Logical Models

Guidelines for Analyzing an Existing Logical Model

Validate the Accuracy of Documentation

Validate the accuracy of documentation for an existing logical model

Reverse Engineer the Model

Reverse engineer the logical model if it differs significantly from the documented model

Identifying Problems with a Logical Model

Analyze the model for consistency

Identify anomalies that are caused by insufficient normalization

Existing Logical Model

Discussion: Expanding Legacy Projects

Scenario:

Your development lead wants to add a new functionality to an existing data project. You cannot find any documentation about the design of the project. You cannot locate any change logs, bug logs or bug fixes. You have access to the current users and to the current production database.

Question:

How would you gather the required information to add anew functionality to the existing data project?

Lab 2: Designing a Logical Database Model

• Exercise 1: Creating a Logical Database Model

• Exercise 2: Normalizing the Logical Database Model

Logon Information

Estimated time: 60 minutes

Virtual machine

User name

Password

NYC-SQL1

Administrator

Pa$$w0rd

Lab Scenario

You are a lead database designer at QuantamCorp. You are working on the Human Resources Vacation and Sick Leave Enhancement (HR VASE) project, which is designed to enhance the current HR system of your organization. This system is based on the QuantamCorp2008 sample database in SQL Server 2008.

The main goals of the HR VASE project are as follows:

• Provide managers with current and historical information about employee vacation and sick-leave data.

• Provide permission to individual employees to view their vacation and sick-leave balances.

• Provide permission to selected employees in the HR department to view and update employee vacation and sick-leave data.

• Provide permission to the HR manager to view and update all the data.

• Standardize employee job titles.

The management has provided you with a Requirements document. The Requirements document contains information about business requirements, cost benefits, availability and reliability for business needs, security features, and performance requirements. The Requirements document also contains statements about how the HR department wants to store information about employee vacation and sick-leave hours.

In lab 1, you created a conceptual model based on the Requirements document provided by the management. In this lab, you will build a logical model based on the conceptual model created earlier, and normalize its entities.

Lab Review

• Why do we need an intermediate table between the entities to support many-to-many relationships?

• How can we determine the column that can be used as a primary key?

• What are the key considerations while selecting a candidate for normalization?

• Why do you use composite primary keys very often in normalized tables?

Module Review and Takeaways

• Review Questions

• Real-world Issues and Scenarios

• Additional Reading Material