THE DATABASE MANAGEMENT SYSTEM CONCEPT - Fundamentals of Database Management Systems - page 51

Sales file

Customer Customer

Customer

Number Name Address

2746795 John Jones 123 Elm Street

Accounts Receivable file

Customer Customer Customer

Number Name Address

2746795 John Jones 123 Elm Street

Credit file

Customer Customer Customer

Number Name Address

FIGURE3.5
Three files with redundant data

2746795 John Jones 123 Elm Street

address data. Often, the solution to this multiple need is redundant data. The sales
department has its own stored file that, among other things, contains the customer
name and address, and likewise for the accounts receivable and credit departments,
Figure 3.5.
One day customer John Jones, who currently lives at 123 Elm Street, moves
to 456 Oak Street. If his address is updated in two of the files but not the third, then
the company's data is inconsistent, Figure 3.6. Two of the files indicate that John
Jones lives at 456 Oak Street but one file still shows him living at 123 Elm Street.
The company can no longer trust its information system. How could this happen?
It could have been a software or a hardware error. But more likely it was because
whoever received the new information and was responsible for updating one or two
of the files simply did not know of the existence of the third. As mentioned earlier,

Sales file

Customer Customer Customer

Number Name Address

2746795 John Jones 456 Oak Street

Accounts Receivable file

Customer Customer Customer

Number Name Address

2746795 John Jones 456 Oak Street

Credit file

Customer Customer Customer

Number Name Address

FIGURE3.6
Three files with a data integrity problem

2746795 John Jones 123 Elm Street