题解 | SQL 187. 牛客直播各科目平均观看时长

WITH t1 AS (
    SELECT course_id,
           ROUND(AVG(TIMESTAMPDIFF(minute ,in_datetime ,out_datetime)),2)  AS avg_Len
    FROM attend_tb
    GROUP BY course_id
)
SELECT course_name,
       avg_Len
       FROM course_tb a
LEFT JOIN t1
ON a.course_id = t1.course_id
ORDER BY avg_Len DESC
;

# 关键在于 TIMESTAMPDIFF 函数的使用方式技巧。