from后面

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;
SELECT * FROM job_grades;
-- 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_level`
FROM (
    SELECT AVG(salary) ag,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;