7일차 # 4-5, 4-10:

- All group functions ignore null values. 

예외) count(*)

  select count(comm) from emp;

  select count(*) from emp;

  SQL> insert into t1 values (null,null ,null );

  SQL> commit;

  SQL> select * from t1;

  SQL> select count(a), count(b), count(c), count(*) from t1;

  select avg(comm), avg(nvl(comm, 0))

  from emp;