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
示例1

输入

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');

输出

SQL|91.25
R|60.33
Python|58.00
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