SELECT av.*,g.`grade_level` FROM ( SELECT AVG(salary) ag,department_id FROM employees GROUP BY department_id ) av INNER JOIN job_grades g ON av.ag BETWEEN g.`lowest_sal` AND g.`highest_sal`;
mysql from后接子查询，实际为将结果集合并成一张表
原文：https://blog.51cto.com/14437184/2438395
