File and Database Design; Logic Modeling

Class 24

SDLCProject Identification

& Selection

Project Initiation& Planning

Analysis

Logical Design

Physical Design***

Implementation

Maintenance

Databases File Systems Databases

Hierarchical Database Model Network Database Model Relational Database Model Object Oriented Database Model

File Systems Each new application is designed

with its own set of files. Problems:

Changes in files require changes to programs.

Uncontrolled redundancy Inconsistent data Limited data sharing

Database Approach Negatives

Need new, specialized personnel

Need for explicit backups

Interference due to shared data

Organizational conflict

Postives Minimal data

redundancy Consistent data Integration of data Sharing of data Data independence Ease of application

development Reduced program

maintenance

Designing a relational database Create a table (file) for each entity type. Choose a primary key for each table. Choose appropriate data types and value

restrictions for each field. Create new tables to represent many-to-many

relationships. Add foreign keys to represent one-to-many

relationships. Define referential integrity constraints. Evaluate schema quality and make necessary

improvements.

What we’re looking at Databases, Tables (files, entities),

Records, Fields (attributes)

How we’ll define above in the Data Dictionary

Tables Each entity on your ER diagram

will become a table in your database.

Records A row in the table. A group of fields (or attributes)

stored in adjacent memory locations and retrieved together as a unit.

Fields May also be called an attribute or

data element.

All fields (or attributes on your ER diagram) will be defined in the Data Dictionary.

Fields Name Data type Primary key(s) Data integrity Handling missing data Ownership

Data Integrity Default value Picture control Range controls Null Value controls

Data Dictionary The repository of all data

definitions for all organizational applications.

What we’ll show in a DD: Table (file) (entity) name All fields (attributes) within the

table. For each field:

Name Note if primary key Definition/description of field Data type (page 352, or whether its

numeric or character plus the length)

TABLE: PIG

Primary Key Attribute

NameAttribute Definition Attribute Type

* PigID The unique identifier of the pig

5 numeric characters

  PigName The name of the pig 15 alpha characters

  PigWeight The weight, in pounds and ounces of the pig

6 numeric characters, with 2 numbers after the decimal

  PigGender The gender of the pig – either male or female

1 alpha character, M or F are allowed

Logic Modeling Knowing what’s going on inside the

programs. What is the logic to create the reports, calculated fields?

We’ll show logic modeling through use of decision trees and decision tables.

Decision Tables A matrix representation of the

logic of a decision, which specifies the possible conditions for the decision and the resulting actions. Condition stubs Action stubs Rules

Steps in creating Decision tables: Name all conditions and the values each

condition can assume. Name all possible actions that can occur. List all possible rules based on every

possible combination of conditions. Define the actions for each rule. Simplify the decision table by removing

“indifferent conditions”.

Examples!

Decision Trees A graphical representation of a

decision situation in which decision situation points (nodes) are connected together by arcs (one for each alternative on a decision) and terminate in ovals (the action that is the result of all of the decisions made on the path leading to the oval

Decision trees Often used for statistical reasons –

for calculating probabilities and making choices based on probabilities.

Used more often for “simpler” problems.

Examples!