如何计算每天发给不同人的邮件数量？

Question

我有一张这样的桌子：

// messages
+----+----------------+-----------+-------------+-------------+
| id |    content     | sender_id | receiver_id |  date_time  |
+----+----------------+-----------+-------------+-------------+
| 1  | whatever1      | 1         | 3           | 1521097240  |
| 2  | whatever2      | 3         | 1           | 1521097241  |
| 3  | whatever3      | 1         | 3           | 1521097242  |
| 4  | whatever4      | 1         | 4           | 1521097243  |
| 5  | whatever5      | 1         | 5           | 1521097244  |
| 6  | whatever6      | 5         | 1           | 1521097245  |
+----+----------------+-----------+-------------+-------------+

现在，我需要计算用户sender_id = 1发送给其他人的消息数。 因此，假设所有这些行都在前一天，则结果应为3 。 因为sender_id = 1已将消息发送到sender_id = 1 receiver_ids = 3,4,5 。 如何在Mysql中进行计数？

---

这是我尝试过的：

SELECT count(1) as sent_messages_num
FROM messages
WHERE sender_id = 1  
AND date_time > UNIX_TIMESTAMP(DATE_SUB(now(), INTERVAL 1 DAY))

因此，我只需要添加分组即可。 但是不确定我该怎么做？

Answer 1

尝试使用以下查询：

SELECT count(*) as sent_messages_num, receiver_id
FROM users
WHERE sender_id = 1  
AND date_time > UNIX_TIMESTAMP(DATE_SUB(now(), INTERVAL 1 DAY))
GROUP BY receiver_id

这将为您提供每个接收者的消息数量。 再次阅读答案后，我认为以下内容可能更合适：

SELECT count(DISTINCT receiver_id) as sent_messages_num
FROM users
WHERE sender_id = 1  
AND date_time > UNIX_TIMESTAMP(DATE_SUB(now(), INTERVAL 1 DAY))

Answer 2

您需要使用distinct的COUNT

SELECT count(distinct receiver_id) as sent_messages_num
FROM users
WHERE sender_id = 1  
AND date_time > UNIX_TIMESTAMP(DATE_SUB(now(), INTERVAL 1 DAY))