SQL使用得到正確的數據集彙總查詢

Question

所以我有用戶，通話，短信表，現在我想呼入和呼出電話的數量和每個用戶短信

我有這個疑問

select u.username, 
     sum(case when c.type = 'incoming' then 1 else 0 end) as incoming, 
     sum(case when c.type = 'incoming' and status = 'true' then 1 else 0 end) as answered, 
     sum(case when c.type = 'outgoing' then 1 else 0 end) as outgoing, 
     sum(case when s.type = 'in' then 1 else 0 end) as sms 
from user u 
join 
     calls c 
     on u.id = c.user_id 
join 
     sms s 
     on u.id = s.user_id 

group by u.username; 

，其結果是這

傳入和在呼叫傳出的是正確的，但在SMS中的結果是錯誤的用戶樣品中，它應爲1列SMS而不是4

Answer 1

要爲避免您對連接所產生的乘法效果，您可以嘗試分別按用戶聚合調用表和短信表，然後加入以下子查詢：

SELECT 
    u.username, 
    COALESCE(t1.incoming, 0) AS incoming, 
    COALESCE(t1.answered, 0) AS answered, 
    COALESCE(t1.outgoing, 0) AS outgoing, 
    COALESCE(t2.sms, 0) AS sms 
FROM user u 
LEFT JOIN 
(
    SELECT 
     user_id, 
     SUM(CASE WHEN type = 'incoming' THEN 1 ELSE 0 END) AS incoming, 
     SUM(CASE WHEN c.type = 'incoming' AND status = 'true' 
       THEN 1 ELSE 0 END) AS answered, 
     SUM(CASE WHEN c.type = 'outgoing' THEN 1 ELSE 0 END) AS outgoing 
    FROM calls 
    GROUP BY user_id 
) t1 
    ON u.id = t1.user_id 
LEFT JOIN 
(
    SELECT 
     user_id, 
     SUM(CASE WHEN s.type = 'in' THEN 1 ELSE 0 END) AS sms 
    FROM sms 
    GROUP BY user_id 
) t2 
    ON u.id = t2.user_id