Geography Reference
In-Depth Information
one record on each line, usually encoded in the ASCII format. Each entry
(e.g., a person's name and address), was separated by a special mark and
commas or tabs separated the characteristics (e.g., name, street, house num-
ber, city, post code, state, and country). Finding a particular person required
searching through the entire data file one entry after another. A flat-file
database can also be represented as a list, a table, or a spreadsheet. In other
types of databases the data is stored as records and fields that correspond to
entries and characteristics in the flat-file database. Records and fields have
become the accepted terms when working with databases. The term “tuple”
is used to represent a single data item in a table. A “field” refers to the divi-
sion of the data into separate parts of each data item. An “attribute” is the
particular entry in a field (e.g., Main Street in the field “Street”). A single
database record including all attributes is called an “entity.”
The relational database sets itself apart from flat-file database through
the way it stores data and the possibilities for relating data. A relational data-
base stores data in separate files, usually called “tables,” which can be related
to other tables in the database by common fields. A relational database may
consist of 100s or even 1000s of tables. Every table in a relational database
has a key field that allows each record to be uniquely identified. This key
field is usually indexed to speed up operations. This key field is especially
important for geographic information because of the large amounts of data
that easily come together. In a relational database, attributes can stand in dif-
ferent relationships to attributes in other tables. A one-to-one relationship
relates a single record in one table with a single record in another table. A
one-to-many relationship relates a single record in one table with multiple
records in another table; a many-to-one relationship does the opposite. A
many-to-many relationship relates many records from one table to many
records from another table. This last relationship is rarely desirable because
the meaningful relationships between the records cannot be differentiated
from spurious and erroneous relationships. A one-to-many or many-to-one
relationship may be called for in a variety of situations (e.g., the cities of one
Simplified representation showing the most common types of databases for geo-
graphic information and maps.
Search WWH ::
Custom Search