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题目：

我的答案：

select tag,CONCAT(avg_play_progress,'%') as avg_play_progress
from (
select tag,
round(avg(if(TIMESTAMPDIFF(SECOND,start_time,end_time)>duration,1,TIMESTAMPDIFF(SECOND,start_time,end_time)/duration))*100,2) as avg_play_progress
from 
tb_video_info as t2
join 
tb_user_video_log as t1
on t1.video_id=t2.video_id ) as t3
group by tag
having avg_play_progress>60
order by avg_play_progress desc;

>>>result

正确答案：

SELECT tag, CONCAT(avg_play_progress, "%") as avg_play_progress
FROM (
    SELECT tag,
        ROUND(AVG(
            IF(TIMESTAMPDIFF(SECOND, start_time, end_time) > duration, 1,
               TIMESTAMPDIFF(SECOND, start_time, end_time) / duration)
        ) * 100, 2) as avg_play_progress
    FROM tb_user_video_log
    JOIN tb_video_info USING(video_id)
    GROUP BY tag
    HAVING avg_play_progress > 60
    ORDER BY avg_play_progress DESC
) as t_progress;

🤔：为什么将要表合并之后再进行group by,having得到的结果不正确？

解释✅：avg一定要使用分组，因为avg是聚合函数，筛选出来的就只有一个结果

源代码：

输入：

DROP TABLE IF EXISTS tb_user_video_log, tb_video_info;
CREATE TABLE tb_user_video_log (
    id INT PRIMARY KEY AUTO_INCREMENT COMMENT '自增ID',
    uid INT NOT NULL COMMENT '用户ID',
    video_id INT NOT NULL COMMENT '视频ID',
    start_time datetime COMMENT '开始观看时间',
    end_time datetime COMMENT '结束观看时间',
    if_follow TINYINT COMMENT '是否关注',
    if_like TINYINT COMMENT '是否点赞',
    if_retweet TINYINT COMMENT '是否转发',
    comment_id INT COMMENT '评论ID'
) CHARACTER SET utf8 COLLATE utf8_bin;

CREATE TABLE tb_video_info (
    id INT PRIMARY KEY AUTO_INCREMENT COMMENT '自增ID',
    video_id INT UNIQUE NOT NULL COMMENT '视频ID',
    author INT NOT NULL COMMENT '创作者ID',
    tag VARCHAR(16) NOT NULL COMMENT '类别标签',
    duration INT NOT NULL COMMENT '视频时长(秒数)',
    release_time datetime NOT NULL COMMENT '发布时间'
)CHARACTER SET utf8 COLLATE utf8_bin;

INSERT INTO tb_user_video_log(uid, video_id, start_time, end_time, if_follow, if_like, if_retweet, comment_id) VALUES
  (101, 2001, '2021-10-01 10:00:00', '2021-10-01 10:00:30', 0, 1, 1, null),
  (102, 2001, '2021-10-01 10:00:00', '2021-10-01 10:00:21', 0, 0, 1, null),
  (103, 2001, '2021-10-01 11:00:50', '2021-10-01 11:01:20', 0, 1, 0, 1732526),
  (102, 2002, '2021-10-01 11:00:00', '2021-10-01 11:00:30', 1, 0, 1, null),
  (103, 2002, '2021-10-01 10:59:05', '2021-10-01 11:00:05', 1, 0, 1, null);

INSERT INTO tb_video_info(video_id, author, tag, duration, release_time) VALUES
  (2001, 901, '影视', 30, '2021-01-01 7:00:00'),
  (2002, 901, '美食', 60, '2021-01-01 7:00:00'),
  (2003, 902, '旅游', 90, '2020-01-01 7:00:00');