orcale：day1
计算函数自动屏蔽null值，不计算null值进计数器
SQL> group by 的增强
SQL> select deptno,job,sum(sal) from emp group by deptno,job
SQL> +
SQL> select deptno,sum(sal) from emp group by deptno
SQL> +
SQL> select sum(sal) from emp
SQL>
SQL> ====
SQL> select deptno,job,sum(sal) from emp group by rollup(deptno,job)
SQL>
SQL> 抽象
SQL> group by rollup(a,b)
SQL> =
SQL> group by a,b
SQL> +
SQL> group by a
SQL> +
SQL> 没有group by
SQL> */
SQL> select deptno,job,sum(sal) from emp group by rollup(deptno,job)