Some Concepts That Underpin GIS - Introducing Geographic Information Systems with ArcGIS - page 24

As a second example, consider how you might store telephone numbers associated with names and
addresses. You could order the list alphabetically, so you could easily find a phone number, knowing
the name. Or you could order the list numerically by telephone number, so you could find a name, given
a number.
A different sort of data structure is “hierarchical.” You could use this approach to store the names and
positions of people in a corporation or a military organization based on who reports to whom. Likewise
you could store voting districts within counties within states. In the computer world, this
“tree-structured directory” approach is used to organize the folders and files contained on a hard disk
drive on a computer. Any folder on the disk may store other folders and files.
Many schemes exist to store information in the memory of a computer or on its secondary devices,
such as disk drives or tapes. The primary method used to store large amounts of information is called
the “relational database,” or RDB, developed by E. F. Codd. 21 The software is described as a RDBMS
(relational database management system). The idea is simple: use a set of two-dimensional tables; for
a given table there is a prescribed format in which the rows relate to entities (objects, people, things in
general), while the columns relate to attributes (characteristics, properties) of entities. The intersection of
a given row and a given column is a cell, containing a value, which defines the particular attribute of the
particular entity. See Figure 1-10.
You can use a database table to store names, occupations, and pay schedules of employees, as in Figure 1-11.
Another example shows part of a database of automobiles registered in a state. Each row would represent
one car; each column would represent one property of cars. See Figure 1-12.
Here is some terminology: The structure that contains the entity, the row, is also called a tuple or record .
The structure that contains the attributes, the column, is also called a field or an item .
Relational Database Nomenclature
Columns consist of “Items” or “Fields.”
Columns have names.
Month
Rows consist of
“Records” or
“Instances.”
A cell contains a value appropriate to its column and row.
FIGURE 1-10 Components of a relational database table
21 Edgar F. Codd was the originator of the relational approach to database management, which he introduced in 1970.
For this work, he was the recipient of the Association for Computer Machinery (ACM) Turing award in 1981.