All Powder Board and Ski

Microsoft Access WorkbookChapter 3: Database TablesJerry PostCopyright © 2007

DBDesign: An Expert System

http://time-post.com/dbdesign Benefits

Makes it easy to create database diagrams Saves data in central location, so changes can be

made from almost any computer Provides immediate detailed feedback on the

design Requirements

Instructors must ask for a free account Instructors and students need a Java-enabled Web

browser

Identifying Key Columns

If you are uncertain about which columns to key. Write them down and evaluate the business rules.

OrderID CustomerID

For a given order, can there ever be more than one customer?If yes, then key CustomerID.In most businesses, only one customer per order, so do not key it.

For a given customer, can there ever be more than one order?If yes, then key OrderID, otherwise, do not key it.All businesses hope to get more than one order from a customer, so OrderID must be key. Underline it. Since OrderID is the only key,these columns belong in the Order table.

CustomerOrder(OrderID CustomerID, … )

AutoNumber

Customer Skill Level

CustomerID, LastName, … Style, SkillLevel

CustomerID, LastName, … Style, SkillLevel

Business rule: Each customer can have one skill in many styles.Business rule: Each style can apply to more than one customer.Need a table with both attributes as keys.

CustomerID, LastName, … Style, SkillLevel

But you cannot include LastName, FirstName and so on, because then you would have to re-enter that data for each customer skill.

Consider what happens if you (incorrectly) try to place Style and SkillLevel in the Customer table:

Customer Style Skills

Customer

CustomerIDLastNameFirstNamePhoneAddressCityStateZIP

CustomerSkill

CustomerIDStyleSkillLevel

Style

StyleStyleDescription

SkillLevel

SkillLevelSkillDescription

Action

ActionCreate Customer table in Design viewEnter column namesSelect data typesAssign the primary keySave the table

Creating Tables in Access

Primary key

Data type

Additional data type info

Column name

Data Types: Subtypes

Selected column

Number data type

Subtype: Single

Action

ActionSelect the Gender columnValidation rule: “Female” Or “Male” Or “Unidentified”Validation text: Please enter Female, Male, or Unidentified

Constraints

Selected column

Acceptable values

Message

Action

ActionSelect the Gender columnSelect the Lookup tabDisplay Control: Combo BoxRow Source Type: Value ListRow Source: “Female”;”Male”;”Unidentified”Switch to datasheet view and test it

Simple Lookup Lists

Selected column

Combo box

Value list

Values in quotes and separated by commas

Lookup tab

Database Table

Lookup listAutoNumber generated

Action

ActionTools/Relationships (or button)Add all tablesMove and resize them to fitDrag and drop key columns on foreign keysCheck integrity and cascade boxesDefine all relationships

Define Relationships

Relationship line

Check all three boxes

Add tables

Drag column and drop

Verify both columns

Create Tables with SQL

CREATE TABLE Customer (CustomerID Long,LastName Text(50),FirstName Text(50),Phone Text(50),Email Text(150),Address Text(50),State Text(50),ZIP Text(15),Gender Text(15),DateOfBirth Date,CONSTRAINT pk_Customer PRIMARY KEY (CustomerID)

)

Create Relationships with SQL

CREATE TABLE Rental (RentID Long,RentDate Date,CustomerID Long,ExpectedReturn Date,PaymentMethod Text(50)CONSTRAINT pk_Rental PRIMARY KEY (RentID)CONSTRAINT fk_RentalCustomer FOREIGN KEY (CustomerID)REFERENCES Customer(CustomerID)

)

ON DELETE CASCADEON UPDATE CASCADE

Note: ON DELETE AND ON UPDATE are not supported with Access 2002, but hopefully will be supported with 2003.

Estimating Database Size

CustomerID Long 4LastName Text(50) 30FirstName Text(50) 20Phone Text(50) 24Email Text(150) 50Address Text(50) 50State Text(50) 2ZIP Text(15) 14Gender Text(15) 10DateOfBirth Date 8

Average bytes per customer 212Customers per week (winter) *200Weeks (winter) *25Bytes added per year 1,060,000

Data Assumptions

200 customers per week for 25 weeks

2 skills per customer

2 rentals per customer per year

3 items per rental

20 percent of customers buy items

4 items per sale

100 manufacturers

20 models per manufacturer

5 items (sizes) per model

Action

ActionCreate a spreadsheetEnter table names as rowsAdd columns for: Bytes, Rows, TotalsCalculate the bytes per table rowEstimate the number of rowsCompute the table and overall totals

Database Table Sizes

Table Bytes Rows Comments TotalCustomer 212 5000 200 per week * 25 weeks 1,060,000CustomerSkill 68 10000 2 per customer 680,000Rental 44 10000 2 per customer 440,000RentItem 78 30000 3 items per rental 2,340,000PaymentMethod 20 5 basic methods 100Sale 152 1000 20 pct of customers buy 152,000SaleItem 36 4000 4 items per sale average 144,000Employee 248 35 Est. per year 8,680Department 70 5 basic list 350BindingStyle 120 15 main ones 1,800Manufacturer 262 100 should be enough 26,200ItemModel 174 2000 20 per manufacturer 348,000Inventory 48 10000 5 per model average 480,000ProductCategory 70 25 should be enough 1,750SkiBoardStyle 110 50 should be enough 5,500SkillLevel 74 10 10 main skill levels 740

5,689,120