NULL « PL SQL Data Types « Oracle PL/SQL Tutorial

21.24.1.All logical operations (including NOT) involving NULL always return NULL
21.24.3.In PL/SQL the Boolean expression NULL=NULL evaluates to FALSE.
21.24.4.Short-circuit evaluation with conditional structures.
21.24.5.Compare with NULL value for equality
21.24.6.Compare with NULL value for non-equality
21.24.7.Compare NULL value and OR operator
21.24.8.Compare with NULL value and 'AND' operator
21.24.9.Compare Integer value with NULL value

21.24.10.Use IS NULL in IF statement
21.24.11.Use NVL in IF statement to deal with the NULL value comparison