select * from hr.employees where department_id is null;

SQL> create table demo_tbl (fname varchar2(10),
  2                         lname varchar2(10)
  3                        )
  4  ;

Table created.

SQL> insert into demo_tbl values (null,null);

1 row created.

SQL> select count(*) from demo_tbl;

  COUNT(*)
----------
         1

1 row selected.

SQL> select count(fname) from demo_tbl;

COUNT(FNAME)
------------
           0

1 row selected.

SQL> insert into demo_tbl values ('Ed',null);

1 row created.

SQL> select count(*) from demo_tbl;

  COUNT(*)
----------
         2
1 row selected.

SQL> select count(fname) from demo_tbl;

COUNT(FNAME)
------------
           1

1 row selected.

12.2> with t as (select 1 x from dual union all select null x from dual)
  2  select avg(x) from t;

    AVG(X)
----------
         1

12.2>
12.2>
12.2>
12.2> with t as (select 1 x from dual union all select null x from dual)
  2  select sum(x)/count(*) from t;

SUM(X)/COUNT(*)
---------------
             .5

12.2>