2 COUNT inside a SELECT with LEFT JOIN

Question

我有 2 个 table：

用户：

ID | NAME 
1  | caio 
2  | mike
3  | peter

发送：

ID | ID_SEND | ID_RECEIVE
1  |    1    |     2
2  |    2    |     3
3  |    3    |     2
4  |    2    |     1

每次用户向另一个用户发送卡片（通过电子邮件发送）时，都会提供 table。

问题：我想写一个SELECT知道用户进入了多少次ID_SEND，用户进入了多少次ID_RECEIVE。我试过这样的事情：

SELECT 
    user.email, 
    COUNT(sends.id_receive) AS numberReceive, 
    COUNT(sends.id_send) AS numberSend
FROM
    user
LEFT JOIN 
    sends ON user.id = sends.id_send OR user.id = sends.id_receive
GROUP BY 
    user.email

问题是 ID_SEND 和 ID_RECEIVE 都返回相同的值，两者的总和...我哪里出错了？

Answer 1

LEFT JOIN两次，一次发送，一次接收

SELECT u.email,
       COUNT(r.id_receive) AS numberReceive,
       COUNT(s.id_send) AS numberSend
FROM user u
LEFT JOIN sends r ON u.id = r.id_receive
LEFT JOIN sends s ON u.id = s.id_send
GROUP BY u.email

或者，单个 LEFT JOIN，使用 case 表达式的条件计数：

SELECT u.email,
       SUM(case when u.id = s.id_receive then 1 else 0 end) AS numberReceive,
       SUM(case when u.id = s.id_send then 1 else 0 end) AS numberSend
FROM user u
LEFT JOIN sends s ON u.id IN (s.id_receive, s.id_send) 
GROUP BY u.email

Answer 2

试试这个。

SELECT S.*, 
   R.received 
FROM   (SELECT u.NAME, 
           Count(snd.id) sends 
    FROM   USER u 
           JOIN sends snd 
             ON snd.id_send = u.id 
    GROUP  BY u.NAME) S 
   LEFT JOIN (SELECT u.NAME, 
                     Count(rec.id) received 
              FROM   USER u 
                     JOIN sends rec 
                       ON rec.id_receive = u.id 
              GROUP  BY u.NAME) R 
          ON S.NAME = R.NAME

2 COUNT inside a SELECT with LEFT JOIN

2 COUNT inside a SELECT with LEFT JOIN

mysql

select

sum

left-join