LeetCode MySQL 1204. 最后一个能进入电梯的人（累加/变量/窗口函数）

+-------------+---------+
| Column Name | Type    |
+-------------+---------+
| person_id   | int     |
| person_name | varchar |
| weight      | int     |
| turn        | int     |
+-------------+---------+
person_id 是这个表的主键。
该表展示了所有等待电梯的人的信息。
表中 person_id 和 turn 列将包含从 1 到 n 的所有数字，
其中 n 是表中的行数。

Queue 表
+-----------+-------------------+--------+------+
| person_id | person_name       | weight | turn |
+-----------+-------------------+--------+------+
| 5         | George Washington | 250    | 1    |
| 3         | John Adams        | 350    | 2    |
| 6         | Thomas Jefferson  | 400    | 3    |
| 2         | Will Johnliams    | 200    | 4    |
| 4         | Thomas Jefferson  | 175    | 5    |
| 1         | James Elephant    | 500    | 6    |
+-----------+-------------------+--------+------+

Result 表
+-------------------+
| person_name       |
+-------------------+
| Thomas Jefferson  |
+-------------------+

为了简化，Queue 表按 turn 列由小到大排序。
上例中 George Washington(id 5), 
John Adams(id 3) 
和 Thomas Jefferson(id 6) 将可以进入电梯,
因为他们的体重和为 250 + 350 + 400 = 1000。
Thomas Jefferson(id 6) 是最后一个体重合适并进入电梯的人。

# Write your MySQL query statement below
select person_name from
(
    select q1.person_name, sum(q2.weight) totalw
    from Queue q1, Queue q2
    where q1.turn >= q2.turn # 小于该turn的全部 sum w
    group by q1.turn
    having(totalw <= 1000)
    order by q1.turn desc
    limit 1
) t

select person_name from
(
    select person_name, @pre := @pre + weight as totalw
    from Queue, (select @pre := 0) tmp
    order by turn
) t
where totalw <= 1000
order by totalw desc
limit 1

select person_name from
(
    select person_name, sum(weight) over(order by turn) totalw
    from Queue
) t
where totalw <= 1000
order by totalw desc
limit 1