1. Retail Sales is used to illustrate a first dimensional model
Chapter 2
Retail Sales is used to illustrate a first dimensional model
Design process
Case study: POS example
Star schema
Facts
Dimensions
Creating the schema in SQL Server
January 2004
Ron McFadyen

2. The Dimensional Design Process
4 Step Dimensional Design Process
Select the business process, examples: invoicing, orders, inventory, general ledger, …
Declare the grain. Determine exactly what an individual fact table row represents. Examples: a line item on an order, a boarding pass to get on a flight, a student’s course registration, a monthly snapshot for a bank account.
Choose the dimensions that apply to the facts. What describes each fact. Examples: customer dimension, student dimension, course dimension, day dimension.
Identify the numeric facts that appear in the rows of the fact table.
January 2004
Ron McFadyen

3. The business process: POS retail sales
Case Study
Case Study
The business process: POS retail sales
Grain of the fact table: individual line items on a POS transaction
The dimensions: date, product, store, promotion
The facts: sales quantity, cost dollar amount, sales dollar amount, gross profit dollar amount (derivable)
January 2004
Ron McFadyen

4. A typical drawing seen in practice, in articles, …
Case Study Schema
Date
Product
Sales facts
Store
Promotion
A typical drawing seen in practice, in articles, …
January 2004
Ron McFadyen

5. Case Study Schema in Peter Chen Notation1 1
Product
Date n n
Sales facts n n n
Store
Promotion 1 1 1
Sales Transaction
Note: Sales transaction does not appear in text. Later in chapter it is discussed as a degenerate dimension
January 2004
Ron McFadyen

6. Facts can be described as additive, non-additive, semi-additive.
Case Study Fact Table
Sales facts
Sales quantity
Sales dollar amount
Cost dollar amount
Gross profit dollar amount
Additive
Facts can be described as additive, non-additive, semi-additive.
Additive: can be meaningfully summed across all dimensions
Semi-additive: …………………….. across some dimensions
Non-additive: can’t be …
The text discusses some non-additive facts that might be included in such a fact table: gross margin, unit price
January 2004
Ron McFadyen

7. Case Study Fact Table The physical table: Sales facts Date key (FK)
Product key (FK)
Store key (FK)
Promotion key (FK)
POS Transaction Number (degenerate dimension)
Sales quantity
Sales dollar amount
Cost dollar amount
Gross profit dollar amount PK
January 2004
Ron McFadyen

8. Case Study Date Dimension
Very descriptive
Easy to set criteria for queries
Easy to get headings for reports
One row for each day (this is the grain of the Date dimension)
PK is a surrogate key
Used in every star schema
Hierarchies are present
Not normalized
attribute hierarchy
Calendar week  …
Fiscal week  …
Date key (PK)
Date
Full date description
Day of week
Day number in epoch
Week number in epoch
Month number in epoch
Day number in calendar month … ….
Last day in week indicator
Holiday indicator
Weekday indicator
SQL date stamp …
Calendar week
Calendar month
Calendar year
Fiscal week
Fiscal month
Fiscal year
January 2004
Ron McFadyen

9. Case Study Product Dimension
Very descriptive
Easy to set criteria for queries
Easy to get headings for reports
One row for each product for sale, or ever sold, by the company
PK is a surrogate key. We do not use the operational PK here. Over time it may not be unique: the business may re-use keys, companies merge …
Not normalized
An attribute hierarchy
Brand  category  department
Product key (PK)
Product description
SKU number
Brand description
Category description
Department description
Package type description
Package size
Fat content
Diet type
Weight
Weight units of measure …
January 2004
Ron McFadyen

10. Case Study Drilling Down/Up
Product
Run a query to generate:
Department, sales amount, sales quantity
Now, add another attribute at a ‘lower’ level such as brand:
Department, brand, sales amount, sales quantity
What is meant by row-headers (in the text)?
Product key (PK)
Product description
SKU number (natural key)
Brand description
Category description
Department description
Package type description
Package size
Fat content
Diet type
Weight
Weight units of measure …
January 2004
Ron McFadyen

11. Case Study Store Dimension
Very descriptive
Easy to set criteria for queries
Easy to get headings for reports
One row for each store
PK is a surrogate key.
Not normalized
An attribute hierarchies
city  county  state  zip
district  region
How does the text handles the “First open date” attribute?
Store key (PK)
Store name
Store number (natural key)
Store street address
Store city
Store county
Store state
Store zip code …
Total square footage
First open date
January 2004
Ron McFadyen

12. Case Study Promotion Dimension
Very descriptive
Easy to set criteria for queries
Easy to get headings for reports
One row for each promotion
PK is a surrogate key.
Need a special row for “no promotion in effect”
Why?
Promotion key (PK)
Promotion name
Price reduction type
Promotion media type
Ad type …
January 2004
Ron McFadyen