Java Reference
In-Depth Information
The use of a high-level language to manipulate the data at the logical level is an
important feature, providing a level of abstraction which lets the user insert or retrieve
data from the tables based on attributes of the data rather than its physical structure.
For example, rather than requiring the user to retrieve a number stored in a certain
location on disk, the use of a high-level query language allows the user to request the
checking balance of a particular customer's account by account number or customer
name.
A further advantage of this approach is that, while the user defines his or her requests
in logical terms, the database management system (DBMS) can implement them in a
highly optimized manner with respect to the physical implementation of the storage
system. By decoupling the logical operations from the physical operations, the DBMS
can achieve a combination of user friendliness and efficiency that would not
otherwise be possible.
Codd's Rules
When Codd initially presented his paper, the meaning of the relational model he
described was not widely understood. To clarify his ideas, Codd published his famous
Fidelity Rules, which are summarized in
Table 1-1
. In theory, a RDBMS must conform
to these rules. As it turns out, some of these rules are extremely difficult to implement
in practice, so no existing RDBMS complies fully.
Table 1-1: Codd's Rules
Rule
Name
Description
0
Foundation Rule
A RDBMS must use its relational facilities exclusively to
manage the database.
1
Information Rule
All data in a relational database must be explicitly
represented at the logical level as values in tables and in
no other way.
2
Guaranteed Access Rule
Every data element must be logically accessible through
the use of a combination of its primary key name,
primary key value, table name, and column name.
3
Systematic Nulls Rule
The RDBMS is required to support a representation of
missing and inapplicable information that is systematic,
distinct from all regular values, and independent of data
type.
4
Dynamic Catalog Rule
The database description or catalog must also be stored
at the logical level as tabular values. The relational
language must be able to act on the database design in
the same manner in which it acts on data stored in the
