题解 | 每个创作者每月的涨粉率及截止当前的总粉丝量

WITH play_info AS(
    SELECT 
        u.video_id,
        DATE_FORMAT(u.start_time, "%Y-%m") AS month,
        u.if_follow,
        CASE WHEN u.if_follow = 0 THEN 0
            WHEN u.if_follow = 1 THEN 1
            WHEN u.if_follow = 2 THEN -1
        END AS fans,
        v.author
    FROM tb_user_video_log u
    LEFT JOIN tb_video_info v USING (video_id)
    WHERE YEAR(u.start_time) = 2021
),
monthly_fans AS (
    SELECT
        month,
        author,
        SUM(fans) AS month_fans,
        COUNT(*) AS play_times
    FROM play_info
    GROUP BY author, month
)

SELECT 
    author, 
    month,
    ROUND(month_fans / play_times, 3) AS fans_growth_rate,
    SUM(month_fans) OVER (
        PARTITION BY author 
        ORDER BY month
        ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW
    ) AS total_fans  -- 截止当月的总粉丝量（累计求和）
FROM monthly_fans
GROUP BY author, month
ORDER BY author, total_fans

这里学到了一个窗口函数的新用法，累计求和：

• ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW：从最早月份累加到当前月。