Database Reference
In-Depth Information
Figure 2-1.
Levels of DBS Architecture
2.1.1 External Level
The
external level
is concerned with individual user views. It therefore varies according to
users' perspectives. The external level is defined by the
external schema
.
Typically, the database is accessed through its external schema. The application
programmer uses both the
host language
and the
data sublanguage
(DSL) to create a user
interface that end users use to access the system:
•
The DSL is the language that is concerned specifically with
database objects and operations. To illustrate, SQL (
structured
query language
) is the industry's standard DSL. Other examples
of data sub-languages are QUEL and KQL (
knowledge query
language
). These languages will be further discussed later in
the course.
