小让の码场SQL187 牛客直播各科目平均观看时长
题目描述
牛客某页面推出了数据分析系列直播课程介绍。用户可以选择报名任意一场或多场直播课。
已知课程表course_tb如下（其中course_id代表课程编号，course_name表示课程名称，course_datetime代表上课时间）：
| course_id | course_name | course_datetime |
|---|---|---|
| 1 | Python | 2021-12-1 19:00-21:00 |
| 2 | SQL | 2021-12-2 19:00-21:00 |
| 3 | R | 2021-12-3 19:00-21:00 |
上课情况表attend_tb如下（其中user_id表示用户编号、course_id代表课程编号、in_datetime表示进入直播间的时间、out_datetime表示离开直播间的时间）：
| user_id | course_id | in_datetime | out_datetime |
|---|---|---|---|
| 100 | 1 | 2021-12-01 19:00:00 | 2021-12-01 19:28:00 |
| 100 | 1 | 2021-12-01 19:30:00 | 2021-12-01 19:53:00 |
| 101 | 1 | 2021-12-01 19:00:00 | 2021-12-01 20:55:00 |
| 102 | 1 | 2021-12-01 19:00:00 | 2021-12-01 19:05:00 |
| 104 | 1 | 2021-12-01 19:00:00 | 2021-12-01 20:59:00 |
| 101 | 2 | 2021-12-02 19:05:00 | 2021-12-02 20:58:00 |
| 102 | 2 | 2021-12-02 18:55:00 | 2021-12-02 21:00:00 |
| 104 | 2 | 2021-12-02 18:57:00 | 2021-12-02 20:56:00 |
| 107 | 2 | 2021-12-02 19:10:00 | 2021-12-02 19:18:00 |
| 100 | 3 | 2021-12-03 19:01:00 | 2021-12-03 21:00:00 |
| 102 | 3 | 2021-12-03 18:58:00 | 2021-12-03 19:05:00 |
| 108 | 3 | 2021-12-03 19:01:00 | 2021-12-03 19:56:00 |
请你统计每个科目的平均观看时长（观看时长定义为离开直播间的时间与进入直播间的时间之差，单位是分钟），输出结果按平均观看时长降序排序，结果保留两位小数。
| course_name | avg_Len |
|---|---|
| SQL | 91.25 |
| R | 60.33 |
| Python | 58.00 |
SQL Schema
sql
drop table if exists course_tb;
CREATE TABLE course_tb(
course_id int(10) NOT NULL,
course_name char(10) NOT NULL,
course_datetime char(30) NOT NULL);
INSERT INTO course_tb VALUES(1, 'Python', '2021-12-1 19:00-21:00');
INSERT INTO course_tb VALUES(2, 'SQL', '2021-12-2 19:00-21:00');
INSERT INTO course_tb VALUES(3, 'R', '2021-12-3 19:00-21:00');
drop table if exists attend_tb;
CREATE TABLE attend_tb(
user_id int(10) NOT NULL,
course_id int(10) NOT NULL,
in_datetime datetime NOT NULL,
out_datetime datetime NOT NULL
);
INSERT INTO attend_tb VALUES(100, 1, '2021-12-1 19:00:00', '2021-12-1 19:28:00');
INSERT INTO attend_tb VALUES(100, 1, '2021-12-1 19:30:00', '2021-12-1 19:53:00');
INSERT INTO attend_tb VALUES(101, 1, '2021-12-1 19:00:00', '2021-12-1 20:55:00');
INSERT INTO attend_tb VALUES(102, 1, '2021-12-1 19:00:00', '2021-12-1 19:05:00');
INSERT INTO attend_tb VALUES(104, 1, '2021-12-1 19:00:00', '2021-12-1 20:59:00');
INSERT INTO attend_tb VALUES(101, 2, '2021-12-2 19:05:00', '2021-12-2 20:58:00');
INSERT INTO attend_tb VALUES(102, 2, '2021-12-2 18:55:00', '2021-12-2 21:00:00');
INSERT INTO attend_tb VALUES(104, 2, '2021-12-2 18:57:00', '2021-12-2 20:56:00');
INSERT INTO attend_tb VALUES(107, 2, '2021-12-2 19:10:00', '2021-12-2 19:18:00');
INSERT INTO attend_tb VALUES(100, 3, '2021-12-3 19:01:00', '2021-12-3 21:00:00');
INSERT INTO attend_tb VALUES(102, 3, '2021-12-3 18:58:00', '2021-12-3 19:05:00');
INSERT INTO attend_tb VALUES(108, 3, '2021-12-3 19:01:00', '2021-12-3 19:56:00');答案
sql
SELECT ct.course_name, ROUND(AVG(TIMESTAMPDIFF(MINUTE, at.in_datetime, at.out_datetime)), 2) AS `avg_Len`
FROM attend_tb at
INNER JOIN course_tb ct USING (course_id)
GROUP BY ct.course_name
ORDER BY avg_Len DESC;