SQL189 牛客直播各科目同时在线人数

SQL189 牛客直播各科目同时在线人数

参考相似题目：SQL88 输出播放量最高的视频-CSDN博客

思路完全一样。。

WITH
    -- 生成所有事件点（开始+1，结束-1）
    events AS (
        SELECT
            course_id,
            in_datetime AS event_time,
            1 AS event_type
        FROM
            attend_tb
        UNION ALL
        SELECT
            course_id,
            out_datetime AS event_time,
            -1 AS event_type
        FROM
            attend_tb
    ),
    -- 按时间排序并计算累计同时播放数
    running_counts AS (
        SELECT
            course_id,
            event_time,
            SUM(SUM(event_type)) OVER (
                PARTITION BY
                    course_id
                ORDER BY
                    event_time ROWS UNBOUNDED PRECEDING
            ) AS concurrent_play
        FROM
            events
        GROUP BY
            course_id,
            event_time
    ),
    -- 找出每个视频的最大同时播放数
    max_concurrent AS (
        SELECT
            course_id,
            MAX(concurrent_play) AS max_num
        FROM
            running_counts
        GROUP BY
            course_id
    )
SELECT
    course_id,
    course_name,
    max_num
FROM
    max_concurrent
    join course_tb using (course_id)
ORDER BY
    course_id