Database Reference
In-Depth Information
13.1.3 Usefulness and Manipulation of Logical Views
Logical views are very useful, particularly during development of software systems that
access related databases. The following are some advantages of views:
•
Provision of some amount of logical data independence in the
face of restructuring of the database
•
Facilitation of assorted external views of the data stored
•
Simplification of the perception of end users — the users
concentrate only on data that is of concern to them
•
Aiding system security — users have access only to data that
concerns them, and cannot access or manipulate in any way, data
to which they are not authorized
Data manipulation operations on
updateable views
are converted to equivalent
operations on the underlying base table(s). In the case of data retrieval (via the
Select
statement), the conversion is straightforward and traces directly to operations on the
underlying base relation(s). In the case of data changes (insertion, update or deletion),
more care is required, as explained below.
A logical view is
updateable
if it meets all of the following criteria:
•
The view is defined to include attributes that constitute a
candidate key of the underlying base relation
•
The view does not involve a JOIN*, UNION or INTERSECT
operation
•
The
Select
statement does not contain the keyword DISTINCT
•
The
Select
statement does not include use of any aggregate
function, a
Group-By-Clause
or a
Having-Clause
•
The view does not contain derived columns
•
The view does not include the
READ ONLY
option
13.2 System Security
In a database system (and many software applications), there are three possible levels of
system security:
•
Access to the system
•
Access to the system resources
•
Access to system data
