Compare with NULL value and 'AND' operator : NULL « PL SQL Data Types « Oracle PL/SQL Tutorial

SQL> SET ECHO ON
SQL> SET SERVEROUTPUT ON
SQL> DECLARE
  2    a     INTEGER;
  3    b     BOOLEAN;
  4    n     INTEGER;     --this will be our null value.
  5  BEGIN
  6    a := 2;
  7
  8    --TRUE and NULL = NULL
  9    IF (a = 2) AND (a <> n) THEN
 10      DBMS_OUTPUT.PUT_LINE('TRUE and NULL = TRUE');
 11    ELSE
 12      DBMS_OUTPUT.PUT_LINE('TRUE and NULL = NULL');
 13    END IF;
 14  END;
 15  /
TRUE and NULL = NULL

PL/SQL procedure successfully completed.

SQL>

21.24.NULL
	21.24.1.	All logical operations (including NOT) involving NULL always return NULL
	21.24.2.	variable:=nvl(value1,value2);
	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