Oracle Database Handling NULL values
Introduction
A column is NULL when it has no value, regardless of the data type of that column. A column should never be compared to NULL using this syntax
a = NULL as the result would be UNKNOWN. Instead use a IS NULL or a IS NOT NULL conditions. NULL is not equal to NULL. To compare two expressions where null can happen, use one of the functions described below. All operators except concatenation return NULL if one of their operand is NULL. For instance the result of 3 * NULL + 5 is null.
Remarks
NULL can't appear in columns restricted by a PRIMARY KEY or a NOT NULL constraint. (Exception is a new constraint with NOVALIDATE clause)
