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Summary
One of the definitions of a database is "a
collection of objects and processes for their manipulation." In a relational
database, the objects are created through either standard SQL99 or
vendor-specific SQL. The
CREATE
<object> syntax can be very
complicated and usually belongs to the database administrator's domain;
nevertheless, understanding how database objects are created is
important.The most important objects introduced in
this chapter include table, index, view, materialized view, synonym, schema,
and sequence. This list by all means does not cover all objects that could be
created within RDBMS using SQL.
The central and most important database
object is table — a logical concept of a data holder
implemented in a database file structure. Tables can be temporary or permanent,
according their life span within an RDBMS. Even such a basic concept can be
implemented differently by the different vendors. Most differences occur in the
context of the
CREATE syntax employed in temporary tables,
whereas the syntaxes used in permanent tables tend to adhere more closely to
the SQL standard.Indexes are not part
of the SQL standard specification and are hidden from users. The purpose of an
index is to speed up data retrieval from a table. A view
differs from a table by being populated with data on demand from its base
tables (with the exception of materialized views); views are defined by an SQL
SELECT query and are dependent on the
tables this query specifies. Views are used for many reasons, explained in the
chapter. Synonyms are used to
facilitate database object referencing by replacing long, fully qualified names
with easy-to-use names.Some other database objects mentioned in
the chapter are domains, tablespaces,
filegroups, and constraints.Each vendor has chosen to implement a
different subset of database objects in its RDBMS software; some of these
objects are not part of the standard SQL, and some of the higher-level
standard-mandated objects are not implemented. Syntax for creating these
objects also differs among the vendors and should be checked against vendors'
documentation.
