簡單的SQL內部聯接查詢 - 無法正常工作

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 

當前使用上述查詢，我​​可以從當前活動拍賣（拍賣＃4）中獲得4個隨機項目。但他們目前都以相同的最高出價金額（來自內部聯合）回來。每個項目應該有自己的最高出價金額（來自內部加入） - 除非該項目沒有出價，那麼它應該是0（或某物）

我在做什麼錯在這裏？ 由於我希望外部部分中的每件商品的最高出價，因此投標表對於放置的每個出價都有一行，因此LIMIT 1和內部聯合中的desc順序排列。 （如果出價是）。

謝謝

Answer 1

嘗試一個子查詢。如果由於​​3210而在投標表中沒有條目，則您創建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

像這樣？

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 

將您的表架構的問題將有助於

編輯：

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 

在這裏你去，包括未投標的項目。

讓我知道，如果我有錯小提琴：http://sqlfiddle.com/#!9/0eae7/2

Answer 3

有幾個問題：

• 按Hockenberry，交叉連結到派生表LIMIT 1每次都會重複剛纔的出價最高，更糟的是，它沒有被過濾，所以它將是任何拍賣的最高出價，如果該行不是針對當前的auction_item_id，那麼它將被連接。將LIMIT移動到外部查詢。
• 即使沒有出價，您也會希望使用LEFT JOIN返回一行。
• 您還需要INNER JOIN的auction_item到item（您最初的查詢將無法運行）

SELECT ai.auction_item_id， ai.starting_bid， COALESCE（b.bid_amount，0）如BID_AMOUNT， 島* FROM auction_item艾 INNER JOIN 項作爲我 ON i.id = ai.listings_id LEFT JOIN（ SELECT auction_item_id，BID_AMOUNT FROM出價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）

編輯

道歉，起初，我不明白你試圖實現。如果你只想要拍價最高（或零，如果沒有出價）任何4個隨機auction_items，這裏要做到這一點的一種方式，用2個派生表：

• 確定4個隨機項目
• 查找的最高價每個auction_items
• LEFT JOIN兩個派生表一起

。

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