Information Technology Reference
In-Depth Information
Figure 5.11
NORTHWESTERN MANUFACTURING
A Typical Data Dictionary Entry
PREPARED BY:
D. BORDWELL
DATE:
04 AUGUST 2007
APPROVED BY:
J. EDWARDS
DATE:
13 OCTOBER 2007
VERSION:
3.1
PAGE:
1 OF 1
DATA ELEMENT NAME:
PARTNO
DESCRIPTION:
OTHER NAMES:
INVENTORY PART NUMBER
PTNO
VALUE RANGE:
100 TO 5000
DATA TYPE:
NUMERIC
POSITIONS:
4 POSITIONS OR COLUMNS
For example, the information in a data dictionary for the part number of an inventory
item can include the following data:
•
Name of the person who made the data dictionary entry (D. Bordwell)
•
Date the entry was made (August 4, 2007)
•
Name of the person who approved the entry (J. Edwards)
•
Approval date (October 13, 2007)
•
Version number (3.1)
•
Number of pages used for the entry (1)
•
Part name (PARTNO)
•
Part names that might be used (PTNO)
•
Range of values (part numbers can range from 100 to 5,000)
•
Type of data (numeric)
•
Storage required (four positions are required for the part number)
A data dictionary is valuable in maintaining an efficient database that stores reliable in-
formation with no redundancy, and makes it easy to modify the database when necessary.
Data dictionaries also help computer and system programmers who require a detailed de-
scription of data elements stored in a database to create the code to access the data.
Storing and Retrieving Data
One function of a DBMS is to be an interface between an application program and the
database. When an application program needs data, it requests the data through the DBMS.
Suppose that to calculate the total price of a new car, an auto dealer pricing program needs
price data on the engine option—six cylinders instead of the standard four cylinders. The
application program requests this data from the DBMS. In doing so, the application program
follows a logical access path. Next, the DBMS, working with various system programs, ac-
cesses a storage device, such as disk drives, where the data is stored. When the DBMS goes
to this storage device to retrieve the data, it follows a path to the physical location (physical
access path) where the price of this option is stored. In the pricing example, the DBMS might
go to a disk drive to retrieve the price data for six-cylinder engines. This relationship is shown
in Figure 5.12.
This same process is used if a user wants to get information from the database. First, the
user requests the data from the DBMS. For example, a user might give a command, such as
LIST ALL OPTIONS FOR WHICH PRICE IS GREATER THAN 200 DOLLARS. This
is the logical access path (LAP). Then, the DBMS might go to the options price section of a
disk to get the information for the user. This is the physical access path (PAP).
Two or more people or programs attempting to access the same record in the same
database at the same time can cause a problem. For example, an inventory control program
might attempt to reduce the inventory level for a product by ten units because ten units were
just shipped to a customer. At the same time, a purchasing program might attempt to increase
