from后面的子查询使用

将子查询结果充当一张表，要求必须起别名

案例：查询每个部门的平均工资的工资等级
1.查询每个部门的平均工资

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

2.连接1的结果集和job_grades表，筛选条件平均工资between lowest_sal and highest_sal

SELECT ag_dep.*,g.'grade_lever'
FROM(
	SELECT AVG(salary),department_id
	FROM employees
	GROUP BY department_id
	) ag_dep
INNER JOIN job_grades g
ON ag_dep.ag BETWEEN lowest_sal AND highest_sal