如何计算每天发给不同人的邮件数量？

Question

I have a table like this:

// 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  |
+----+----------------+-----------+-------------+-------------+

Now I need to count the number of messages that user sender_id = 1 to different people. So assuming all those row are in the past day, then the result should be 3 . Because sender_id = 1 have sent messages to receiver_ids = 3,4,5 . How can I do that count in Mysql?

---

Here is what I've tried:

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

So all I need it adding a grouping. But not sure how should I do that?

Answer 1

Try using the following query:

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

This will give you the number of messages per receiver. After I read the answer again, I think the following might suit better:

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

You need to use distinct in 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))