variable:=nvl(value1,value2); : NULL « PL SQL Data Types « Oracle PL/SQL Tutorial

If the value1 is not NULL, then return it; otherwise return the value2.

You can use expressions, variables, functions, and literals in NVL, as long as both variables are of the same datatype

SQL> declare
  2       v NUMBER;
  3  begin
  4       v:=nvl(v,0)+1;
  5       DBMS_OUTPUT.put_line('New value: <'||v||'>');
  6  end;
  7  /
New value: <1>

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