You can assign NULL as a value for any datatype. The NULL value introduces what
is called three-state logic to your SQL operators. A normal comparison has only two
states: TRUE or FALSE. If you??™re making a comparison that involves a NULL value,
there are three logical states: TRUE, FALSE, and neither.
None of the following conditions are true for Column A if the column contains a
NULL value:
A > 0
A < 0
A = 0
A != 0
The existence of three-state logic can be confusing for end users, but your data may
frequently require you to allow for NULL values for columns or variables.
90 | Chapter 4: Oracle Data Structures
Youhave to test for the presence of a NULL value with the relational operator IS
NULL, since a NULL value is not equal to 0 or any other value. Even the expression:
NULL = NULL
will always evaluate to FALSE, since a NULL value doesn??™t equal any other value.
Basic Data Structures
This section describes the three basic Oracle data structures: tables, views, and
indexes. This section also discusses partitioning, which affects the way that data in
tables and indexes is stored.
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