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_name
FROM
    employees AS e,departments AS d,locations AS l
WHERE
    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 d
WHERE
    l.`location_id`=d.`location_id`

GROUP BY
    country_id
HAVING
    COUNT(*) >2;