列出在每个(每个是关键字,对此group by)部门工作的员工数量、平均工资和平均服务期限。(经典的group by用法) **select deptno,count(*),avg(a.sal),avg(sysdate-HIREDATE) from emp a group by deptno; select deptno,count(*),avg(a.sal),avg(to_number(to_char(sysdate,'yyyy'),'9999')-to_number(...