Geography Reference
In-Depth Information
state or the states of one country). These usually reflect a hierarchy or a
grouping of attributes and their corresponding records.
The relational database has several advantages for geographic represen-
tation and cartographic representation. First, the conceptual model of the
database is distinct from the physical model (how the database is stored and
managed on computer hardware). Second, separate tables help maintain the
integrity of the potential meaning of database elements. Most relational data-
bases now use structured query language (SQL) for constructing queries
involving tables of a single database, or with tables in other databases, even
on other computers. Third, the clarity of the relationships aids people using
the database with previous experiences of the database. Reliable processing
is critical for queries of geographic information and online maps. Fourth, it
is possible to define multiple views of the same data in different database
tables (e.g., listing entries by street address or alphabetically by name).
While the relational database is the most common type of database and
possibly the only type of database you will ever work with, two other types of
databases may be significant. The first of these is a hierarchical database.
This database is organized by defining a hierarchy into which all data is
stored (e.g., country, state/province, county, municipality). This type of data-
base was frequently used for business transactions, but is being replaced by
relational databases. The second is the object-oriented database. In this data-
base, data is stored as objects that not only include characteristics, but also
possible actions. The objects in an object-orientated database exist only when
the database program is running on a computer (fortunately their character-
istics can be stored for later use). Objects act on other objects, receiving and
sending messages and processing data. For example, an object-oriented data-
base of sewers may consist of objects with information about the size of the
sewer pipe, but also how much water can flow through the pipe in a minute,
and what happens to excess water. Additionally, a sewer pipe may have
attributes that can be passed on to other sewer pipe records in the database,
or modified to reflect the characteristics of another sewer pipe. Some GIS
already use object-oriented databases.
Database table relates (or joins) are made by identifying the same attributes in the key fields from
two separate tables.
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