sql 练习题

有两张表，如下

　　t_person, 员工表，包含年龄，姓名，以及部门id

　　t_department，部门表，包含部门id，部门名称

题1：获取每个部门的平均年龄

SELECT
    d.id,
    d. NAME,
    p.avg_age
FROM
    t_department d
LEFT JOIN (
    SELECT
        avg(age) avg_age,
        department_id
    FROM
        t_person
    GROUP BY
        department_id
) p ON p.department_id = d.id

题2：获取年龄最大的员工（可能不止一个）

select * from t_person where age = (select max(age) from t_person);

题3：获取每个部门年龄最大的员工

SELECT
    *
FROM
    t_person p
JOIN (
    SELECT
        max(age) max_age,
        department_id
    FROM
        t_person
    GROUP BY
        department_id
) t ON p.department_id = t.department_id
AND p.age = t.max_age;

题4：获取每个部门大于平均年龄的员工

SELECT
    *
FROM
    t_person p
JOIN (
    SELECT
        avg(age) avg_age,
        department_id
    FROM
        t_person
    GROUP BY
        department_id
) t ON p.department_id = t.department_id
AND p.age >= t.avg_age;