1.使用以下语句：

SELECT MAX(salary),MIN(salary),AVG(salary),SUM(salary),job_id
FROM employees
GROUP BY job_id
ORDER BY job_id

结果如下：

2.使用以下语句：

SELECT MAX(salary)-MIN(salary) AS DIFFERENCE
FROM employees

结果如下：

3.使用以下语句：

SELECT MIN(salary),manager_id
FROM employees
WHERE manager_id IS NOT NULL
GROUP BY manager_id
HAVING MIN(salary)>=6000;

结果如下：

4.使用以下语句：

注意：count(*)是用来统计表中记录的一个函数，返回匹配条件的行数。

SELECT department_id,COUNT(*),AVG(salary)
FROM employees
WHERE department_id is not null
GROUP BY department_id
ORDER BY AVG(salary) DESC

结果如下：

5.使用以下语句：

SELECT COUNT(*),job_id
FROM employees
GROUP BY job_id