Question

我不确定如何从连接表中返回最高值。表中当前有4个项目，但查询只返回一个。 MAX（b.bid）正在打破查询。

SELECT i.id, 
 i.user_id, 
 i.item_title, 
 i.item_description, 
 i.item_condition, 
 i.active, 
 i.add_date, 
 MAX(b.bid)
FROM items i
LEFT JOIN bids b
 ON i.id = b.item_id
WHERE i.user_id = ? AND i.active = 1 
ORDER BY i.id DESC

Answer 1

你可能想要这样的东西：

SELECT i.id, 
 i.user_id, 
 i.item_title, 
 i.item_description, 
 i.item_condition, 
 i.active, 
 i.add_date, 
 b.bid
FROM items i
LEFT JOIN (SELECT MAX(bid) AS bid, item_id 
           FROM bids 
           GROUP BY item_id) b
 ON i.id = b.item_id
WHERE i.user_id = ? AND i.active = 1 
ORDER BY i.id DESC

您获得bid每item_id的最大值，然后执行LEFT JOIN操作。

Answer 2

如果我理解你的问题，你需要一个group by条款：

SELECT i.id, 
 i.user_id, 
 i.item_title, 
 i.item_description, 
 i.item_condition, 
 i.active, 
 i.add_date, 
 MAX(b.bid)
FROM items i
LEFT JOIN bids b
 ON i.id = b.item_id
WHERE i.user_id = ? AND i.active = 1 
group by i.id, 
 i.user_id, 
 i.item_title, 
 i.item_description, 
 i.item_condition, 
 i.active, 
 i.add_date
ORDER BY i.id DESC

Answer 3

你可以这样做：

SELECT i.id, 
 i.user_id, 
 i.item_title, 
 i.item_description, 
 i.item_condition, 
 i.active, 
 i.add_date, 
 b.bid
FROM items i
LEFT JOIN 
(
    SELECT
        MAX(bids.bid) as bid,
        bids.item_id
    FROM
        bids
    GROUP BY
        bids.item_id
) as b
 ON i.id = b.item_id
WHERE i.user_id = ? AND i.active = 1 
ORDER BY i.id DESC

Answer 4

如果您想要每个项目max(bid)，那么您需要逐项group查询。

SELECT i.id, 
 i.user_id, 
 i.item_title, 
 i.item_description, 
 i.item_condition, 
 i.active, 
 i.add_date, 
 MAX(b.bid)
FROM items i
LEFT JOIN bids b
 ON i.id = b.item_id
WHERE i.user_id = ? AND i.active = 1 
GROUP BY i.id
ORDER BY i.id DESC

此结果取决于每个出价的所有其他字段是相同的，它看起来应该是这样。这是对group by的mysql扩展的有效使用。否则，您需要在group by子句中的select中指定每个字段。

Mysql - 从连接表返回最大值

4 个答案: