數查詢由不同的外鍵

Question

加入了兩次參考假設我有兩個表：

Users 
id: integer 
name: string 

Messages 
id: integer 
sender_id: integer 
receiver_id: integer 
body: string 

我需要發送和接收信息的統計爲所有用戶。

發送郵件時我的查詢按預期工作：

SELECT users.id, COUNT(*) total_sent 
FROM users 
LEFT OUTER JOIN messages sent_messages ON sent_messages.sender_id = users.id 
GROUP BY users.id; 

+----+------------+ 
| id | total_sent | 
+----+------------+ 
| 1 |   3 | 
| 2 |   2 | 
| 3 |   1 | 
+----+------------+ 

但是我需要擴大查詢，並在同一時間獲得接收的消息的數量。我的嘗試是：

SELECT users.id, COUNT(sent_messages.id) total_sent, COUNT(received_messages.id) total_received 
FROM users 
LEFT OUTER JOIN messages sent_messages ON sent_messages.sender_id = users.id 
LEFT OUTER JOIN messages received_messages ON received_messages.receiver_id = users.id 
GROUP BY users.id; 

不幸的是，它會產生錯誤的結果。

Answer 1

你可以指望不同receiver_id總髮送的消息和獨特的sender_id收到信息

SELECT u.id, COUNT(distinct s.receiver_id) total_sent, 
COUNT(distinct r.sender_id) total_received 
FROM users u 
LEFT OUTER JOIN messages s ON s.sender_id = u.id 
LEFT OUTER JOIN messages r ON r.receiver_id = u.id 
GROUP BY u.id; 

DEMO

Answer 2

我希望這將幫助你：

select u.id, r.total_received, s.total_sent from users as u left outer join (select count(id) as total_received, receiver_id from Messages group by receiver_id) as r on r.receiver_id =u.id left outer join (select count(id) as total_sent, sender_id from Messages group by sender_id) as s on sender_id=u.id