Oracle Data Structures - Oracle Essentials - page 102

to somehow determine whether that value has actually been assigned or simply exists
as a replacement for NULL.
If you choose not to use NULL values, you're forcing a value to be assigned to a column
for every row. You are, in effect, eliminating the possibility of having a column that
doesn't require a value, as well as potentially assigning misleading values for certain
columns. This situation can be misleading for end users and can lead to inaccurate
results for summary actions such as AVG (average).
Avoiding NULL values simply replaces one problem—educating users or providing
them with an interface that implicitly understands NULL values—with another set of
problems, which can lead to a loss of data integrity.
Basic Data Structures
This section describes the three basic Oracle data structures: tables, views, and indexes.
This section discusses partitioning, which affects the way that data in tables and indexes
is stored.This section also covers editions , a new type of table introduced in Oracle
Database 11 g Release 2.
Tables
The table is the basic data structure used in a relational database. A table is a collection
of rows. Each row in a table contains one or more columns . If you're unfamiliar with
relational databases, you can map a table to the concept of a file or database in a non‐
relational database, just as you can map a row to the concept of a record in a nonrela‐
tional database.
As of Oracle9 i , you can define external tables . As the name implies, the data for an
external table is stored outside the database, typically in a flat file. The external table is
read-only; you cannot update the data it contains. The external table is good for loading
and unloading data to files from a database, among other purposes.
Oracle Database 11 g introduced the ability to create virtual columns for a table. These
columns are defined by an expression and, although the results of the expression are
not stored, the columns can be accessed by applications at runtime. Oracle Database
12 c introduces the invisible column, which is stored and maintained like a regular col‐
umn but is not accessible by user request or considered by the query optimizer.
Editions
Oracle Database 11 g Release 2 introduced a new concept called an edition . An edition
is simply a version of a table, and a table can have more than one edition simultaneously.
Editions are used to implement edition-based redefinition (EBR), useful as table struc‐
tures evolve over time.