求平均值的时候，分母到底该如何选择？

–求平均值的时候，分母到底该如何选择？ 如果记录有NULL，则不把NULL，算入总数之内。
select AVG(commission_pct) from employees;
1、select (select sum(commission_pct) from employees)/(select count(*) from employees) from dual ?
还是
2、select (select sum(commission_pct) from employees)/(select count(commission_pct) from employees) from dual ?
答案是第二个
那么
select AVG(NVL(commission_pct,0)) from employees;又是多少呢？
答案与第一个相同