我的SELECT是否產生任何東西？

Question

我的SELECT有什麼問題嗎？它應該計算每天不同IP的數量，然後計算總和。這是查詢：

SELECT SUM(NumIpsPerDay) AS user_hits 
FROM 
    (SELECT DATE(tracking.timestamp) AS DayTrackCount, 
      COUNT(DISTINCT tracking.ip) AS NumIpsPerDay 
    FROM USER, 
     tracking 
    GROUP BY DATE(tracking.timestamp) 
    WHERE tracking.user_id = $user_id 
    GROUP BY $user_id LIMIT 1) 

我在看它，我覺得它可能是日期。我的SELECT有什麼不對嗎？

Answer 1

SELECT SUM(NumIpsPerDay) AS user_hits 
FROM 
(
    SELECT DATE(tracking.timestamp) AS DayTrackCount, 
      COUNT(DISTINCT tracking.ip) AS NumIpsPerDay 
    FROM user, tracking 
    WHERE tracking.user_id = $user_id 
    GROUP BY DATE(tracking.timestamp) 
) x 

Answer 2

試試這個：

SELECT SUM(tab.NumIpsPerDay) AS user_hits 
FROM 
    (SELECT DATE(tracking.timestamp) AS DayTrackCount, 
    COUNT(DISTINCT tracking.ip) AS NumIpsPerDay 
    FROM USER, tracking 
    WHERE tracking.user_id = $user_id 
    GROUP BY DATE(tracking.timestamp)) as tab 

你爲什麼要擺在子查詢日期（tracking.timestamp）？

Answer 3

嗯，是的。您正在執行user和tracking之間的cross join（笛卡爾產品）。但是，您沒有使用user表中的任何字段。你可能根本不需要桌子。

試試這個：

SELECT SUM(NumIpsPerDay) AS user_hits 
FROM (SELECT DATE(tracking.timestamp) AS DayTrackCount, 
      COUNT(DISTINCT tracking.ip) AS NumIpsPerDay 
     FROM tracking 
     WHERE tracking.user_id = $user_id 
     GROUP BY DATE(tracking.timestamp 
    ) t 
GROUP BY $user_id; 

另外：

1. 子查詢沒有一個別名
2. ，則不需要limit的group by
後
3. 的where條款來了