Database Reference
In-Depth Information
The first and last properties are constraints that both end users and software engineers
should be cognizant of, since they have to manage data contained in the database; they are
also of interest to the database designer. The second and third properties on the surface
are immaterial to end users as well as software engineers; they are usually enforced by the
DBMS in a manner that is transparent to the end user. However, when the DBMS is written,
concern has to be given to accessing of records. Further, DBMS suites are typically written to
give the illusion that the attributes of a relation are ordered.
3.3.2 Kinds of Relations
A database will consist of various types of relations, some of them at different stages of the
system. The common categories of relations are mentioned below:
1.
Base Relations are named and permanently represented in
the database. They make up the conceptual schema of the
database; they form the foundation of the database.
2.
Views (virtual relations) are derived from named (base)
relations. A view stores the definition of the virtual relation
(derived from base relations), but stores no physical data. It is
simply a logical (conceptual/external) interpretation of data
stored in base relations. SQL views and System i logical files
are good examples of views.
3.
Snapshots are named, derived relations. They differ from
logical views in that they are represented in terms of definition
as well as physically stored data. From the perspective of the
end user, a snapshot relation is typically (but not necessarily)
read-only. To illustrate, consider two systems — System-A
and System-B — which both need to access a database table,
Table-X. Suppose that System-A has update rights to Table-X,
but System-B does not. Table-X is therefore stored in
System-A's database; a duplicate version for read-only
purposes, is stored in System-B, and is periodically updated
(without user interference) from System-A.
4.
Query Results: Queries are typically entered at a command
prompt (they may be also embedded in high level language
programs or stored in special query files). Results may be
directed to screen, printer, or a named relation. An important
principle to note is that a query when executed always results
in a new relation. This principle will be elucidated later in the
course.
5.
Intermediate Results: The DBMS may create an intermediate
relation to assist in furnishing a final answer to a complex
query request. This will also be elucidated later in the course.
6.
Temporary Relations are named relations that are destroyed
at some point in time.
 
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