Question

我有一个存储用户评论的数据库表：

comments(id, user_id, created_at)

从该表中，我想获得过去7天内第一次评论的用户数。

这是我到目前为止所拥有的：

SELECT COUNT(DISTINCT `user_id`) 
FROM `comments` 
WHERE `created_at` BETWEEN DATE_SUB(NOW(), INTERVAL 7 DAY) AND NOW()

这会给出已评论的用户数量，但不会考虑这些评论是否首先针对其用户。

Answer 1

SELECT COUNT(DISTINCT `user_id`) 
FROM comments c1
WHERE created_at BETWEEN DATE_SUB(NOW(), INTERVAL 7 DAY) AND NOW()
AND NOT EXISTS 
   (SELECT NULL 
   FROM comments c2 
   where c1.user_id = c2.user_id
   AND c2.create_at < DATE_SUB(NOW(), INTERVAL 7 DAY));

Answer 2

SELECT COUNT(DISTINCT user_id)
FROM comments AS c1
WHERE c1.created_at BETWEEN DATE_SUB(NOW(), INTERVAL 7 DAY) AND NOW()
AND NOT EXISTS (SELECT 1 FROM comments AS c2 
                WHERE c2.user_id = c1.user_id AND c2.created_at < c1.created_at)

NOT EXISTS子句检查相同的user_id是否具有早期created_at时间的记录。如果是这样，这意味着这不是他们第一次评论，因此我们应该打折这个记录。

我保留了DISTINCT user_id，因为可能会同时创建两条评论。您也可以尝试使用以下内容，它只为每个用户获取第一个记录，因此您可以取消DISTINCT，但我不知道哪个更优化：

SELECT COUNT(*)
FROM comments AS c1
WHERE c1.created_at BETWEEN DATE_SUB(NOW(), INTERVAL 7 DAY) AND NOW()
AND NOT EXISTS (SELECT 1 FROM comments AS c2 
                WHERE c2.user_id = c1.user_id 
                AND (c2.created_at < c1.created_at 
                    OR (c2.created_at = c1.created_at AND c2.id < c1.id)))

选择给定时间段内类型的第一个记录

2 个答案: