Firebird 2.5:
SQL> select nullif(null,0) f01 from rdb$database;

F01
======
<null>

Oracle 11.2.0.3.0:
SQL> select nullif(null,0) f01 from dual;
select nullif(null,0) f01 from dual
              *
ошибка в строке 1:
ORA-00932: несовместимые типы данных: ожидается -, получено CHAR

MS SQL 2005 Developer edition:

select nullif(null,0) f01
Msg 8133, Level 16, State 1, Line 1
None of the result expressions in a CASE specification can be NULL.

SQL> select nullif( to_number(null),0) f01 from dual;
select nullif( to_number(null),0) f01 from dual
               *
ошибка в строке 1:
ORA-00932: несовместимые типы данных: ожидается -, получено NUMBER


SQL> select nullif( cast(null as number),0) f01 from dual;

       F01
----------


SQL> select case when null=0 then null else null end f01 from dual;

F
-

select nullif( cast(null as int),0) f01

select case when null=0 then null else null end
Msg 8133, Level 16, State 1, Line 1
None of the result expressions in a CASE specification can be NULL.

select case when cast(null as int)=0 then null else null end
Msg 8133, Level 16, State 1, Line 1
None of the result expressions in a CASE specification can be NULL.

select nullif((select null),0)

The type of the first argument to NULLIF cannot be the NULL constant because the type of the first argument has to be known.