Databases Reference
In-Depth Information
Figure 7-1. The three-level structure of a database system
In fact, there are a variety of structures in which the data might be stored, including hash
tables, balanced trees, linked lists, nested records, and so on, and the choice of data
structure is not a simple one. However, I will not pursue a discussion of the physical
database in this topic. Suffice it to say that, at the physical level, the data is viewed as a
structured
collection of bits, and the sole purpose of the structure is to provide efficient
access to the data. The physical level of a database is often referred to as the
internal
level
.
The
conceptual database
is a conceptual view of the database as a whole. It gives the
data a
logical
structure
. For instance, in a relational database system, the data is viewed
as a collection of tables, with column headings describing the attributes of the
corresponding entity class. Moreover, tables are related to one another through certain
columns.
The conceptual model is intended to model the entire database. However, individual users
may be interested in views of only specific portions of the data. For instance, in the
LIBRARY database, a student using the library's online database catalog is probably not
interested in the price of the topic, but is interested in where it is located on the shelves.
Thus, a single database, such as LIBRARY, may need different views for the student than
for the librarian.
The highest level in the three-tier structure consists of the individual
views
of the data
that may be held by users of the database. Views are also referred to as
subschemes
, and
this level of the tier is also referred to as the
external level
.
As another example, we can think of the Microsoft Visual Basic programming language
as providing an external view of the Jet database management system that is geared
toward database programmers. We can think of Microsoft Access as providing an
