chapter 3 requirements and business rules copyright © 2012 pearson education, inc. publishing as...

Chapter 3

Requirements and Business Rules

Copyright © 2012 Pearson Education, Inc. Publishing as Prentice Hall Chapter3.1

Client Server Relations

• Much of software can be divided into one of two types:– Servers– Clients


Servers

• A server is software that offers “services” to other software.

• For instance a Web server provides Web pages that are requested by a browser.

• Databases usually behave as servers.• (Some machines are optimized to host

Server software. They are also commonly referred to as servers).


Clients

• Clients are software that request services.• A browser, for instance, requests a Web

page to load and view.• An application client can request data

from a database.


Client Server Example


Review of the Issues

• Reviewing the issues with the current data management system is a good place to start.

• Several of the requirements of the new database will be to resolve those issues.

• Reviewing the issues also helps you refocus on the “problem domain.”


Problem Domain

• The problem domain represents the business problems a database is meant to solve.

• For a retail sale database, for instance, the problem domain is the sale, and all that is involved with the sale.

• For a science database dealing with earthquakes, the domain would be the locations, sizes and depths of earthquakes.

Copyright © 2012 Pearson Education, Inc. Publishing as Prentice Hall Chapte3.7

Requirements

• It is important to identify all the requirements of the database.

• A requirement represents something the database must store or do.

• There are several types of requirements:– Data Requirements– Report Requirements– Security requirements


Data Requirements

• Data Requirements refer to the attributes the database must store in order to meet the information needs of an organization.

• It is important to identify these data requirements as completely as possible.


Report Requirements• Report requirements refer to the reports

the database will have to generate.• For example, the Tutor database will have

to report on tutor’s hours, the numbers of unduplicated student sessions and the demographics of the students using the tutoring services, among others.

• The data required to generate those reports must be in the database.


Security Requirements• Security requirements refer to the limits that

must be placed on database access in order to protect the integrity and privacy of the data in the database.

• Typically different users of the database will have different levels of access depending on their needs.

• A user should have access to the data and resources they need to do their work, but no more.


Actors and Requirements

• It is useful to think about requirements in terms of each of the actors who will be using the database.

• What does that particular actor need to do with the data in order to do their job?

• What are the limits on what each actor should be able to do?


Access Requirements

• Access is usually defined in terms of what kinds of things a user can do in the database.

• For instance, a user can be given permission to SELECT some data – that is to view it–but not other data.

• They may be given permission to edit certain data, but not all data.


Types of Security RequirementsType of Access Description

Create The permission to make new database objects such as tables or views

Alter The permission to modify database objects

Drop The permission to remove database object

Select The permission to see data in a table or view

Update The permission to modify data in a table

Insert The permission to add data rows to a table

Delete The permission to remove data rows from a table

Execute The permission to run database executables such as stored procedures


Business Rules

• A business rule is a rule about how data is collected, stored or processed.

• Examples of Business rules:– All quarter grades must be between 0 and 4.– No patron can have more than 20 items

checked out at a time.– Payments must be made within 30 days or a

25 dollar late fee will be added.


Enforcing Business Rules

• Some business rules can be enforced in the database itself by placing constraints on the data. (The quarter grade must be between 0 and 4).

• Other business rules must be enforced through other means such as “Triggers.”


Triggers• A trigger is a block of SQL code that is

triggered by an event such as an INSERT, UPDATE or DELETE.

• Triggers can be used to enforce things such as “No patron can check out more that 20 items.”

• When the database inserts a new item the trigger fires and totals the number of unreturned items. If it is greater than 20 it can notify the librarian or the patron.


Reviewing Requirements and Business Rules

• When you have listed all the requirements and business rules you can discover, you should always review them with the chief stakeholders.

• Use the review to make sure you have a complete list of requirements.

• Also make sure you have understood the business rules and processes.


A Little Bit of Grammar

• This is a technique to help identify the data requirements for a database.

• Review all the documents, interviews and questionnaires and list all the major nouns.


Grouping Around Themes

• The next step is to sort the nouns into broad themes or groups.

• These themes may become entities in your database design.

• The other nouns that belong to the those themes will become attributes.


Entities

• Entities are things that the database is concerned with.

• In the tutoring database, for instance major themes are student, class, tutor, session and request.


Attributes

• Attributes represent data that describe entities.

• Attributes of student, for instance, include:– Student ID– Student Name– Student Address– Student phone, etc.


Keys• When you have your preliminary entities

and attributes defined you can start thinking about keys.

• There are several types of keys:– Primary Keys– Candidate Keys– Natural Keys– Composite Keys– Surrogate Keys


Primary Keys• A primary key uniquely identifies a row of

data.• A primary key must be unique for every

row (that is it can never repeat in the table that will result from the entity).

• For instance a student ID can uniquely identify an individual student and the data associated with him or her.

• Every entity should have a primary key.


Candidate Keys

• A candidate key is an attribute or attributes of an entity that have the potential to become a primary key.

• Candidate keys are not actual keys, but are a list of attributes that should be considered when choosing the primary key.


Natural Keys

• There are basically two ways of making keys: natural and surrogate.

• Natural keys are keys formed by using an attribute that “naturally” belongs to the entity, such as a student ID or a phone number.


Composite Keys

• Composite keys are keys composed of more than one attribute.

• For example to get a unique designation of a course section, it is necessary to combine the quarter, the year, and the item number.

• Composite keys are one key made out of many parts.

Copyright © 2012 Pearson Education, Inc. Publishing as Prentice Hall Chapte3.27

Surrogate Keys

• Surrogate keys are keys that have no business meaning.

• Often they are just integers incremented row by row.

• They can also be things such as time stamps of auto generated GUIDs.


Comparison Between Natural and Surrogate Keys: Advantages

Natural Keys• Protect better against

accidently repeating the same information.

• Belong to the entity and tend to make foreign keys more understandable.

Surrogate Keys• Are always unique.• Do not contain any business

logic and are therefore not subject to changes in business rules.

• Are easier to define and use.


Comparison Between Natural and Surrogate Keys: Disadvantages

Natural Key(s)• Uniqueness is hard to

guarantee.• May require clumsy

composite keys consisting of several attributes to be unique.

• Are subject to changes in business rules (think of Social Security numbers, for instance).

Surrogate Keys• Automatically grant

uniqueness to a row making it easier to accidentally insert the same information twice.

• Have no relation to the data making database relations less readable.


Documentation

• All requirements and business rules should be documented clearly and stored.

• Even preliminary definitions of entities, attributes and keys should be kept for later reference.


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Copyright © 2012 Pearson Education, Inc. Publishing as Prentice Hall

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