MySQL之select和from后面的子查询

select 后面(很少用的,可以用前面讲的方法实现)

案例：

select d.*,(
select d.department_name count(*) from employees e
where d.department_id=e.department_id
) from departments d;

from后面的子查询：

案例：查询每个部门的平均工资等级

select avg(salary),department_id from employees group by department_id;

SELECT
avg_res.avgs,avg_res.`department_id`,g.`grade_level`
FROM
(
SELECT AVG(salary) avgs,department_id
FROM employees GROUP BY department_id
) avg_res
,`job_grades` g
WHERE
avg_res.avgs BETWEEN g.`lowest_sal` AND g.`highest_sal`;