如何在MySQL的內部查詢中從外部查詢訪問值

Question

SELECT * FROM events e LEFT JOIN venue v ON v.id=e.venueid 
CROSS JOIN (select (((5*sum(rating = 5))+(4*sum(rating = 4))+(3*sum(rating = 3))+(2*sum(rating = 2))+sum(rating = 1))/count(*)) rating 
from feedbacks where boxid = **e.boxid**) r 
WHERE e.startdate = CURDATE() AND e.starttime >= CURTIME() 
LIMIT 10;

我無法訪問子查詢中的e.boxid 。 有人可以幫忙嗎？

Answer 1

另一種方法使用相關子查詢：

SELECT e.*, v.*,
       (SELECT (((5*sum(rating = 5))+(4*sum(rating = 4))+(3*sum(rating = 3))+(2*sum(rating = 2))+sum(rating = 1))/count(*)) as rating 
        FROM feedbacks f
        WHERE f.boxid = e.boxid
       ) as rating
FROM events e LEFT JOIN 
     venue v
     ON v.id = e.venueid 
WHERE e.startdate = CURDATE() AND e.starttime >= CURTIME() 
LIMIT 10;

在這種情況下，因為WHERE子句過濾掉了許多記錄（可能），相關子查詢可能比GROUP BY更有效。 如果您在feedbacks(boxid)有索引feedbacks(boxid)則尤其如此。

Answer 2

在boxid上使用GROUP BY將它帶到子查詢之外，然后將其與boxid列上的events表連接，如下所示：

SELECT 
    *
FROM
    events e
        LEFT JOIN
    venue v ON v.id = e.venueid
        JOIN
    (SELECT 
        boxid,
            (((5 * SUM(rating = 5)) + (4 * SUM(rating = 4)) + (3 * SUM(rating = 3)) + (2 * SUM(rating = 2)) + SUM(rating = 1)) / COUNT(*)) rating
    FROM
        feedbacks
    GROUP BY boxid) r ON e.boxid = r.boxid
WHERE
    e.startdate = CURDATE()
        AND e.starttime >= CURTIME()
LIMIT 10;

您還可以使用相關的子查詢，如下所示：

SELECT e.*,
  v.*,
  (SELECT (((5*SUM(rating = 5))+(4*SUM(rating = 4))+(3*SUM(rating = 3))+(2*SUM(rating = 2))+SUM(rating = 1))/COUNT(*)) AS rating
  FROM feedbacks f
  WHERE f.boxid = e.boxid
  ) rating
FROM events e
LEFT JOIN venue v
ON v.id           = e.venueid
WHERE e.startdate = CURDATE()
AND e.starttime  >= CURTIME() LIMIT 10;