mysql - GROUP BY MONTH 给出错误的结果

Question

我正在尝试使用以下查询获取按月分组的独特产品 View

SELECT
  Month(timestamp)           AS 'month',
  COUNT(DISTINCT visitor_id) AS 'unique'
FROM productviews pv INNER JOIN products p ON pv.product_id = p.id
WHERE p.vendor_id = 8 AND YEAR(timestamp) = 2018
GROUP BY month(timestamp);

我得到了

+---------+--------+
| month   | unique |
+---------+--------+
|    1    |  3     |
+---------+--------+
|    2    |  10    |
+---------+--------+
|    3    |  2     |
+---------+--------+
|    4    |  4     |
+---------+--------+

但唯一产品浏览总数小于上一次查询的总和

SELECT count(DISTINCT pv.visitor_id)
FROM productviews pv INNER JOIN products p ON pv.product_id = p.id
WHERE p.vendor_id = 8 AND year(timestamp) = 2018

+---------+
|  count  |
+---------+
|    16   |
+---------+

按月分组是否正确？还是我错过了什么？

Answer 1

您的查询没问题。

因为一位访问者可能已经访问了一个多月。