33. 查询 employees 表中有多少个部门select count(distinct department_id)from employees		34. 查询全公司奖金基数的平均值(没有奖金的人按 0 计算)select avg(nvl(commission_pct, 0))from employees		35. 查询各个部门的平均工资--错误: avg(salary) 返回公司平均工资, 只有一个值; 而 department_id 有多个值, 无法匹配返回select department_id, avg(salary)from employees				 **在 SELECT 列表中所有未包含在组函数中的列都应该包含在 GROUP BY 子句中--正确: 按 department_id 进行分组select department_id, avg(salary)from employeesgroup by department_id36. Toronto 这个城市的员工的平均工资SELECT avg(salary)
FROM employees e JOIN departments d
ON e.department_id = d.department_id
JOIN locations l
ON d.location_id = l.location_id
WHERE city = 'Toronto'		37. (有员工的城市)各个城市的平均工资SELECT city, avg(salary)
FROM employees e JOIN departments d
ON e.department_id = d.department_id
JOIN locations l
ON d.location_id = l.location_id
GROUP BY city		38. 查询平均工资高于 8000 的部门 id 和它的平均工资.		SELECT department_id, avg(salary)
FROM employees e 
GROUP BY department_id
HAVING avg(salary) > 8000		39. 查询平均工资高于 6000 的 job_title 有哪些SELECT job_title, avg(salary)
FROM employees e join jobs j
ON e.job_id = j.job_id
GROUP BY job_title
HAVING avg(salary) > 6000

1.	组函数处理多行返回一行吗? 
是
2.	组函数不计算空值吗？
是
3.	where子句可否使用组函数进行过滤? 
不可以，用having替代
4.	查询公司员工工资的最大值，最小值，平均值，总和
a)	select max(salary),min(salary),avg(salary),sum(salary)
b)	from employees
5.	查询各job_id的员工工资的最大值，最小值，平均值，总和
a)	select job_id,max(salary),min(salary),avg(salary),sum(salary)
b)	from employees
c)	group by job_id
6.	选择具有各个job_id的员工人数
a)	select job_id,count(employee_id)
b)	from employees
c)	group by job_id
7.	查询员工最高工资和最低工资的差距（DIFFERENCE）
a)	select max(salary),min(salary),max(salary)-min(salary) "DIFFERENCE"
b)	from employees
8.	查询各个管理者手下员工的最低工资，其中最低工资不能低于6000，没有管理者的员工不计算在内
a)	select manager_id,min(salary)
b)	from employees
c)	where manager_id is not null
d)	group by manager_id
e)	having min(salary) >= 6000
9.	查询所有部门的名字，location_id，员工数量和工资平均值
a)	select department_name,location_id,count(employee_id),avg(salary)
b)	from employees e right outer join departments d
c)	on e.department_id = d.department_id
d)	group by department_name,location_id
10.	查询公司在1995-1998年之间，每年雇用的人数，结果类似下面的格式
total	1995	1996	1997	1998
20	3	4	6	7select count(*) "total",count(decode(to_char(hire_date,'yyyy'),'1995',1,null)) "1995",count(decode(to_char(hire_date,'yyyy'),'1996',1,null)) "1996",count(decode(to_char(hire_date,'yyyy'),'1997',1,null)) "1997",count(decode(to_char(hire_date,'yyyy'),'1998',1,null)) "1998"
from employees
where to_char(hire_date,'yyyy') in ('1995','1996','1997','1998')