子查询及关联查询

经典案例1

#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;

　　on关联条件后面控制关联的范围。