SQL> declare
  2     i integer;
  3  begin
  4     i := case when i is null then 1 end;
  5  end;
  6  /

PL/SQL procedure successfully completed.

SQL> declare
  2     i integer;
  3  begin
  4     i := decode(i,1,2,3);
  5  end;
  6  /
   i := decode(i,1,2,3);
        *
ERROR at line 4:
ORA-06550: line 4, column 9:
PLS-00204: function or pseudo-column 'DECODE' may be used inside a SQL statement only
ORA-06550: line 4, column 4:
PL/SQL: Statement ignored


SQL> declare
  2     i integer;
  3  begin
  4    i := nvl(i,1);
  5  end;
  6  /

PL/SQL procedure successfully completed.

SQL> SELECT banner
  2  FROM   v$version
  3  /

BANNER
----------------------------------------------------------------
Oracle Database 10g Express Edition Release 10.2.0.1.0 - Product
PL/SQL Release 10.2.0.1.0 - Production
CORE    10.2.0.1.0      Production
TNS for 32-bit Windows: Version 10.2.0.1.0 - Production
NLSRTL Version 10.2.0.1.0 - Production

SQL> DECLARE
  2    myvar PLS_INTEGER := 3;
  3    myothervar PLS_INTEGER;
  4  BEGIN
  5    myothervar := DECODE(myvar,2,3,4);
  6  END;
  7  /
  myothervar := DECODE(myvar,2,3,4);
                *
ERROR at line 5:
ORA-06550: line 5, column 17:
PLS-00204: function or pseudo-column 'DECODE' may be used inside a SQL
statement only
ORA-06550: line 5, column 3:
PL/SQL: Statement ignored

SQL> DECLARE
  2    myvar      PLS_INTEGER := 3;
  3    myothervar PLS_INTEGER;
  4  BEGIN
  5    myothervar := NVL(myvar,2);
  6  END;
  7  /

PL/SQL procedure successfully completed.

SQL>
SQL> DECLARE
  2    myvar      PLS_INTEGER := 3;
  3    myothervar PLS_INTEGER;
  4  BEGIN
  5    myothervar := NVL2(myvar,-1,-2);
  6  END;
  7  /
  myothervar := NVL2(myvar,-1,-2);
                *
ERROR at line 5:
ORA-06550: line 5, column 17:
PLS-00201: identifier 'NVL2' must be declared
ORA-06550: line 5, column 3:
PL/SQL: Statement ignored


SQL>
SQL> SELECT NVL2(NULL,1,2) x
  2  FROM   dual
  3  /

         X
----------
         2