我有两个用于购买和用户的表（将包括下面的模式）。我想知道的是一段时间内所有用户的平均购买率。
说清楚点，我不想（总购买量/总用户数），我想知道更多像用户“每月2次购买”
我无法找出正确的算法来使用，虽然我认为这可能取决于购买之间的日期差异。任何关于如何思考这个算法和实际代码的帮助都将受到欢迎
用户表：

ID        NAME        EMAIL      CREATED
2907      Philip      xxxx       Friday, April 6, 2018 14:55 PM
2859      Jane        xxxx       Friday, April 6, 2018 12:36 PM

采购表：

ID        TIMESTAMP                            SKU       USER_ID
1152497   Tuesday, April 10, 2018 12:05 AM     46        98271
1152498   Tuesday, April 10, 2018 12:05 AM     7887      207120

获取“用户id、购买次数和购买金额”的当前代码

SELECT count(*) AS count, sum(b.price) AS sum, u.id AS id, DATEDIFF(MAX(p.timestamp),MIN(p.timestamp)) as date_diff
FROM purchase p
LEFT JOIN book b on p.book_id = b.id
LEFT JOIN user u on p.user_id = u.id
WHERE date(p.timestamp) BETWEEN date(date_add(now(), INTERVAL -365 day)) AND date(date_add(now(), INTERVAL -1 day))
AND u.timestamp BETWEEN date(date_add(now(), INTERVAL -730 day)) AND date(date_add(now(), INTERVAL -365 day))
GROUP BY u.id
ORDER BY count DESC

当前结果

Count     Sum       ID        Date Diff
634       4,240     88,981    226
549       9,432     130,140   337
387       518,009   113,849   320