Databases Reference
In-Depth Information
They offer the standard features and support most, if not all, of the standard
command set, but provide their own extension to the language, giving them
added functionality. These extensions usually take the form of additional com-
mands or additional command options. There are also differences in how fea-
tures that are not fully defined in the SQL standard are implemented.
For example, the SQL standard defines basic database objects such as tables,
views, and indexes. It does not define how these features are physically imple-
mented. Microsoft SQL Server stores database objects in one or more database
files, with each file containing one or more database objects. MySQL, on the
other hand, creates separate files for database objects. Each table in MySQL can
be effectively thought of stored as its own operating system file.
Basic features of SQL can be described as falling within the following categories:
▲
Data definition language (DDL):
statements used to create and maintain
database objects.
▲
Data manipulation language (DML):
statements used to retrieve and
manipulate data.
▲
Command operators:
operator symbols and keywords used to run arith-
metic, comparison, and logical operations.
▲
Functions:
special executables that return values.
▲
Transaction control:
include statements used to initiate and complete or
abort transaction processing.
What this means is that some things that you might have considered part of
SQL because they are common to relational database systems are not. Instead
they are features and command extensions implemented first by one DBMS
provider and sometimes copied by others. For example, the SQL language spec-
ifies nothing about scheduled command execution, but this is a feature sup-
ported by Microsoft SQL Server and many others. The SQL language defines
procedures,
which are compiled sets of executable statements, in the context of
the statements used to create and modify them. It does not include any specific
management procedures to be included with a DBMS, but most do support a
wide array of predefined management procedures that install with the database
server and are treated as part of the vendor's language set.
6.1.2 Using SQL
There are two basic options for executing SQL commands: interactive SQL and
embedded SQL.
Interactive SQL,
also known as
dynamic SQL,
refers to state-
ments that you run directly, interacting with the database server.
Embedded SQL
refers to functionality that is embedded in a procedure or part of an application
written in a different programming language.
