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。
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