这题也是来源牛客。窗口函数做完了，开始聚合函数的题目了。 对试卷得分做min-max归一化_牛客题霸_牛客网 (nowcoder.com)
题干
现有试卷信息表examination_info（exam_id试卷ID, tag试卷类别, difficulty试卷难度, duration考试时长, release_time发布时间）：
| id | exam_id | tag | difficulty | duration | release_time |
|---|---|---|---|---|---|
| 1 | 9001 | SQL | hard | 60 | 2020-01-01 10:00:00 |
| 2 | 9002 | C++ | hard | 80 | 2020-01-01 10:00:00 |
| 3 | 9003 | 算法 | hard | 80 | 2020-01-01 10:00:00 |
| 4 | 9004 | PYTHON | medium | 70 | 2020-01-01 10:00:00 |
试卷作答记录表exam_record（uid用户ID, exam_id试卷ID, start_time开始作答时间, submit_time交卷时间, score得分）：
| id | uid | exam_id | start_time | submit_time | score |
|---|---|---|---|---|---|
| 6 | 1003 | 9001 | 2020-01-02 12:01:01 | 2020-01-02 12:31:01 | 68 |
| 9 | 1001 | 9001 | 2020-01-02 10:01:01 | 2020-01-02 10:31:01 | 89 |
| 1 | 1001 | 9001 | 2020-01-01 09:01:01 | 2020-01-01 09:21:59 | 90 |
| 12 | 1002 | 9002 | 2021-05-05 18:01:01 | (NULL) | (NULL) |
| 3 | 1004 | 9002 | 2020-01-01 12:01:01 | 2020-01-01 12:11:01 | 60 |
| 2 | 1003 | 9002 | 2020-01-01 19:01:01 | 2020-01-01 19:30:01 | 75 |
| 7 | 1001 | 9002 | 2020-01-02 12:01:01 | 2020-01-02 12:43:01 | 81 |
| 10 | 1002 | 9002 | 2020-01-01 12:11:01 | 2020-01-01 12:31:01 | 83 |
| 4 | 1003 | 9002 | 2020-01-01 12:01:01 | 2020-01-01 12:41:01 | 90 |
| 5 | 1002 | 9002 | 2020-01-02 19:01:01 | 2020-01-02 19:32:00 | 90 |
| 11 | 1002 | 9004 | 2021-09-06 12:01:01 | (NULL) | (NULL) |
| 8 | 1001 | 9005 | 2020-01-02 12:11:01 | (NULL) | (NULL) |
在物理学及统计学数据计算时，有个概念叫min-max标准化，也被称为离差标准化，是对原始数据的线性变换，使结果值映射到[0 - 1]之间。
转换函数为：
请你将用户作答高难度试卷的得分在每份试卷作答记录内执行min-max归一化后缩放到[0,100]区间，并输出用户ID、试卷ID、归一化后分数平均值；最后按照试卷ID升序、归一化分数降序输出。（注：得分区间默认为[0,100]，如果某个试卷作答记录中只有一个得分，那么无需使用公式，归一化并缩放后分数仍为原分数）。
由示例数据结果输出如下：
| uid | exam_id | avg_new_score |
|---|---|---|
| 1001 | 9001 | 98 |
| 1003 | 9001 | 0 |
| 1002 | 9002 | 88 |
| 1003 | 9002 | 75 |
| 1001 | 9002 | 70 |
| 1004 | 9002 | 0 |
解释：高难度试卷有9001、9002、9003；
作答了9001的记录有3条，分数分别为68、89、90，按给定公式归一化后分数为：0、95、100，而后两个得分都是用户1001作答的，因此用户1001对试卷9001的新得分为(95+100)/2≈98（只保留整数部分），用户1003对于试卷9001的新得分为0。最后结果按照试卷ID升序、归一化分数降序输出。
测试数据
drop table if exists examination_info,exam_record;
CREATE TABLE examination_info (
id int PRIMARY KEY AUTO_INCREMENT COMMENT '自增ID',
exam_id int UNIQUE NOT NULL COMMENT '试卷ID',
tag varchar(32) COMMENT '类别标签',
difficulty varchar(8) COMMENT '难度',
duration int NOT NULL COMMENT '时长',
release_time datetime COMMENT '发布时间'
)CHARACTER SET utf8 COLLATE utf8_bin;
CREATE TABLE exam_record (
id int PRIMARY KEY AUTO_INCREMENT COMMENT '自增ID',
uid int NOT NULL COMMENT '用户ID',
exam_id int NOT NULL COMMENT '试卷ID',
start_time datetime NOT NULL COMMENT '开始时间',
submit_time datetime COMMENT '提交时间',
score tinyint COMMENT '得分'
)CHARACTER SET utf8 COLLATE utf8_general_ci;
INSERT INTO examination_info(exam_id,tag,difficulty,duration,release_time) VALUES
(9001, 'SQL', 'hard', 60, '2020-01-01 10:00:00'),
(9002, 'C++', 'hard', 80, '2020-01-01 10:00:00'),
(9003, '算法', 'hard', 80, '2020-01-01 10:00:00'),
(9004, 'PYTHON', 'medium', 70, '2020-01-01 10:00:00'),
(9005, 'WEB', 'hard', 80, '2020-01-01 10:00:00'),
(9006, 'PYTHON', 'hard', 80, '2020-01-01 10:00:00'),
(9007, 'web', 'hard', 80, '2020-01-01 10:00:00'),
(9008, 'Web', 'medium', 70, '2020-01-01 10:00:00'),
(9009, 'WEB', 'medium', 70, '2020-01-01 10:00:00'),
(9010, 'SQL', 'medium', 70, '2020-01-01 10:00:00');
INSERT INTO exam_record(uid,exam_id,start_time,submit_time,score) VALUES
(1001, 9001, '2020-01-01 09:01:01', '2020-01-01 09:21:59', 90),
(1003, 9002, '2020-01-01 19:01:01', '2020-01-01 19:30:01', 75),
(1004, 9002, '2020-01-01 12:01:01', '2020-01-01 12:11:01', 60),
(1003, 9002, '2020-01-01 12:01:01', '2020-01-01 12:41:01', 90),
(1002, 9002, '2020-01-02 19:01:01', '2020-01-02 19:32:00', 90),
(1003, 9001, '2020-01-02 12:01:01', '2020-01-02 12:31:01', 68),
(1001, 9002, '2020-01-02 12:01:01', '2020-01-02 12:43:01', 81),
(1001, 9005, '2020-01-02 12:11:01', null, null),
(1001, 9001, '2020-01-02 10:01:01', '2020-01-02 10:31:01', 89),
(1002, 9002, '2020-01-01 12:11:01', '2020-01-01 12:31:01', 83),
(1002, 9004, '2021-09-06 12:01:01', null, null),
(1002, 9002, '2021-05-05 18:01:01', null, null);
题目解析
关于max-min归一化可以参考这个：www.cnblogs.com/chaosimple/…
关于题干说的两次计算都取整，这里只有最后一次计算需要取整，否则最终计算结果会有差异。
解答：
-- 求解每种试卷的最大最小值
select
distinct exam_id,
max(score) over(partition by exam_id) as max_score,
min(score) over(partition by exam_id) as min_score
from exam_record
where exam_id in (select exam_id from examination_info where difficulty='hard')
and score is not null
;
-- 求出每条作答记录归一化后的值
select
tba.uid,
tba.exam_id,
(tba.score-tbb.min_score)/(tbb.max_score-tbb.min_score) * 100 as new_score
from exam_record as tba
inner join
(select
distinct exam_id,
max(score) over(partition by exam_id) as max_score,
min(score) over(partition by exam_id) as min_score
from exam_record
where exam_id in (select exam_id from examination_info where difficulty='hard')
and score is not null
) as tbb
on tba.exam_id=tbb.exam_id
;
然后是最终代码：
select
distinct uid, exam_id,
round(avg(new_score) over(partition by uid, exam_id), 0) as avg_new_score
from
(select
tba.uid,
tba.exam_id,
ifnull((tba.score-tbb.min_score)/(tbb.max_score-tbb.min_score) * 100, score) as new_score
from exam_record as tba
inner join
(select
distinct exam_id,
max(score) over(partition by exam_id) as max_score,
min(score) over(partition by exam_id) as min_score
from exam_record
where exam_id in (select exam_id from examination_info where difficulty='hard')
) as tbb
on tba.exam_id=tbb.exam_id where tba.score is not null) as tb
group by uid, exam_id, new_score
order by exam_id asc, avg_new_score desc
;
