mysql 多表查询案例

#案例：选中city在多伦多工作的员工

SELECT    last_name,job_id,e.department_id,department_nameFROM    employees AS e,departments AS d,locations AS lWHERE    e.`department_id`=d.`department_id`AND    d.`location_id`=l.`location_id`AND    l.city='Toronto'

#案例；查询每个国家下的部门个数大于2的国家编号

SELECT    country_id ,COUNT(*)FROM    locations AS l,departments AS dWHERE    l.`location_id`=d.`location_id`GROUP BY    country_idHAVING    COUNT(*) >2;