Database Reference
In-Depth Information
The user(s) of the database does not need to know how the data is actually stored within
these files, or even which file contains the data item(s) of interest. Inmost organizations,
DBA handles the details of installing and configuring the database software and data files
and making the database available to the database users. The DBMS with the association of
computer's operating system automatically manages the data files, including all file open-
ing, reading, closing, updating and writing operations. The database user should not be re-
quired to refer to physical data files when using a database, which is in sharp contrast with
spreadsheets and word processing, where the user must consciously save the document(s)
and choose file names and storage locations.
2.5.2 The Logical/ conceptual Layer
The logical layer or conceptual layer is the first of two layers of abstraction in the database.
Physical layer has a concrete existence in the operating system files, whereas the logical
layer exists only as abstract data structures assembled from the physical layer as needed.
It describes what data is being store in the database and what kinds of relationships exist
among data. The DBMS transforms the data in the data files into a common structure. This
layer is also called the
schema
. Schema is a collection of all the data items stored in a par-
ticular database. Depending on the particular DBMS, this can be a set of two-dimension-
al tables, a hierarchical structure similar to a company's organization chart, or some other
structure.
2.5.3 The External Layer
The external layer is the second layer of abstraction in the database. This layer is composed
of the user views, which are collectively called the
subschema
. In this layer, users and ap-
plication programs can access the database by connecting and issuing queries (command)
against the database. Ideally, only the DBA deals with the physical and logical layers. The
DBMS handles the transformation of selected items from one or more data structures in the
logical layer to form each user view. The user views can be predefined and stored in the
database for reuse, or they can be temporary items that are built by the DBMS to hold the
results of a single ad hoc database query until no longer needed by the database user.
2.6 Data Independence
Data independence is a form of database management that keeps data separated from all
programs that make use of the data. Data independence ensures that the data cannot be re-
defined or reorganized by any of the programs that make use of the data. In this manner, the
data remains accessible, but is also stable and cannot be corrupted by the applications using
