THE DATA WAREHOUSE - Fundamentals of Database Management Systems - page 348

SALE

Salesperson Produc t Time Perio d

Numbe r Number Numbe r Quantity

1 137 16386 001 57

2 137 24013 001 129

3 137 16386 002 24

4 137 24013 002 30

5 137 16386 102 85

6 137 24013 102 36

7 204 16386 102 111

8 204 24013 102 44

9 137 16386 103 47

10 137 24013 103 59

11 204 16386 103 13

12 204 24013 103 106

13 137 16386 331 63

14 137 24013 331 30

15 186 16386 331 25

16 186 24013 331 16

17 204 16386 331 44

18 204 24013 331 107

F IGURE 13.7 (Continued)
General Hardware Company data
warehouse sample data

19 361 16386 331 18

20 361 24013 331 59

Good Reading Bookstores Data Warehouse
Does Good Reading Bookstores need a data warehouse? Actually, this is a very
good question, the answer to which is going to demonstrate a couple of important
points about data warehouses. At first glance, the answer to the question seems to
be: maybe not! After all, the sales data in Good Reading's transactional database
already carries a date attribute , as shown in the SALE table of Figure 5.16. Thus,
it looks like Good Reading's transactional database is already historical! But Good
Reading does need a data warehouse for two reasons. One is that, while Good
Reading's transactional database performs acceptably with perhaps the last couple
of months of data in it, its performance would become unacceptable if we tried
to keep ten years of data in it. The other reason is that the kinds of management
decision making that require long-term historical sales data do not require daily data.
Data aggregated to the week level is just fine for Good Reading's decision making
purposes and storing the data on a weekly basis saves a lot of time over retrieving
and adding up much more data to answer every query on data stored at the day level.
Figure 13.8 shows the Good Reading Bookstores data warehouse star schema
design. The fact table is SALE and each of its records indicates how many of a
particular book a particular customer bought in a particular week (here again week is
the lowest-level time period) and the price that the customer paid per book. For this