简单的SQL内部联接查询 - 无法正常工作

Question

SELECT ai.auction_item_id, ai.starting_bid, b.bid_amount, i.* 
    FROM 
    auction_item as ai, //Table contains auction specific details about the item 
    item as i //Table contains general details about the item 
    INNER JOIN (
      SELECT auction_item_id, bid_amount 
      FROM bid xb //Table contains bids on item 
      ORDER BY amount DESC 
      LIMIT 1) b 
    ON b.auction_item_id = ai.auction_item_id 
    WHERE 
    ai.auction_id = 4 AND 
    i.id = ai.listings_id 

    ORDER BY RAND() 
    LIMIT 4 

当前使用上述查询，我​​可以从当前活动拍卖（拍卖＃4）中获得4个随机项目。但他们目前都以相同的最高出价金额（来自内部联合）回来。每个项目应该有自己的最高出价金额（来自内部加入） - 除非该项目没有出价，那么它应该是0（或某物）

我在做什么错在这里？ 由于我希望外部部分中的每件商品的最高出价，因此投标表对于放置的每个出价都有一行，因此LIMIT 1和内部联合中的desc顺序排列。 （如果出价是）。

谢谢

Answer 1

尝试一个子查询。如果由于​​3210而在投标表中没有条目，则您创建0。

SELECT 
    ai.auction_item_id, ai.starting_bid, 
    Coalesce((SELECT max(xb.bid_amount) 
       FROM bid xb 
       WHERE xb.auction_item_id = ai.auction_item_id), 0) AS bid_amount, 
    i.* 
FROM auction_item AS ai 
INNER JOIN item AS i ON i.id = ai.listings_id 
WHERE ai.auction_id = 4 
ORDER BY Rand() 

Answer 2

像这样？

SELECT ai.auction_item_id, ai.starting_bid, b.bid_amount, max(bid_amount) 
    FROM auction_item as ai 
     inner join bid b 
     on ai.auction_item_id = b.auction_item_id 
     inner join items i 
     on i.id = ai.listings_id 
    where ai.auction_id = 4 

将您的表架构的问题将有助于

编辑：

SELECT ai.auction_item_id, ai.starting_bid, IFNULL(max(bid_amount), 0) max_bid 
     FROM auction_item as ai 
      left join bid b 
      on ai.auction_item_id = b.auction_item_id 
      inner join items i 
      on i.id = ai.listings_id 
     WHERE ai.auction_id = 4 
     GROUP BY ai.auction_item_id 

在这里你去，包括未投标的项目。

让我知道，如果我有错小提琴：http://sqlfiddle.com/#!9/0eae7/2

Answer 3

有几个问题：

• 按Hockenberry，交叉连结到派生表LIMIT 1每次都会重复刚才的出价最高，更糟的是，它没有被过滤，所以它将是任何拍卖的最高出价，如果该行不是针对当前的auction_item_id，那么它将被连接。将LIMIT移动到外部查询。
• 即使没有出价，您也会希望使用LEFT JOIN返回一行。
• 您还需要INNER JOIN的auction_item到item（您最初的查询将无法运行）

SELECT ai.auction_item_id， ai.starting_bid， COALESCE（b.bid_amount，0）如BID_AMOUNT， 岛* FROM auction_item艾 INNER JOIN 项作为我 ON i.id = ai.listings_id LEFT JOIN（ SELECT auction_item_id，BID_AMOUNT FROM出价XB ORDER BY量DESC）b ON b.auction_item_id = ai.auction_item_id WHERE ai.auction_id = 4 ORDER BY RAND（） LIMIT 4; [SqlFiddle here]（http://sqlfiddle.com/#！9/28922/1）

编辑

道歉，起初，我不明白你试图实现。如果你只想要拍价最高（或零，如果没有出价）任何4个随机auction_items，这里要做到这一点的一种方式，用2个派生表：

• 确定4个随机项目
• 查找的最高价每个auction_items
• LEFT JOIN两个派生表一起

。

SELECT items.auction_item_id, IFNULL(topBids.amount, 0) as topBid 
FROM 
(SELECT auction_item_id 
    FROM auction_item 
    ORDER BY RAND() 
    LIMIT 4 
) items 
LEFT JOIN 
    (SELECT b.auction_item_id, b.amount 
    FROM bid b 
    GROUP BY b.auction_item_id 
    ORDER BY amount DESC) topBids 
ON items.auction_item_id = topBids.auction_item_id; 

Updated Fiddle