语法:
select 分组函数，列（要求出现在group by的后面）
from 表
where ..
group by 分组的列表
order by ...

#案例：查询各个部门的平均工资

SELECT AVG(salary),department_id
FROM employees
GROUP BY
    department_id
ORDER BY
    department_id DESC;

#案例:查询每个工种的最高工资

SELECT
    MAX(salary),job_id
FROM
    employees
GROUP BY
    job_id;

#案例:查询每个部门的位置个数

SELECT
    COUNT(location_id),department_name
FROM
    departments
GROUP BY
    department_name;

#案例:查询邮箱中包含a字符的，每个部门的平均工资

SELECT
    AVG(salary),department_id
FROM
    employees
WHERE
    email LIKE('%a%')
GROUP BY
    department_id;

#案例：查询有奖金的每个领导手下员工的最高工资

SELECT
    MAX(salary),manager_id
FROM 
    employees
WHERE
    commission_pct IS NOT NULL
GROUP BY 
    manager_id;