左連接條件不影響結果

Question

我試圖用一些條件創建一個左連接查詢，但它似乎很奇怪。

SELECT c.country_id, c.country_name, IFNULL(MAX(b.bid_amount),0) 
FROM countries AS c 
LEFT JOIN items_bids AS b 
ON c.country_id = b.bid_country_id 
AND b.bid_removed = 0 
LEFT JOIN advertisers AS a 
ON a.advertiser_id = b.bid_advertiser_id 
AND a.advertiser_balance > 0 
AND a.advertiser_review = 0 
LEFT JOIN items AS n 
ON b.bid_item_id = n.item_id 
AND n.item_approved = 1 
AND n.item_deleted = 0 
AND n.item_paused = 0 
GROUP BY c.country_name 

它具有相同的結果，因爲這：它沒有被任何的ON子句條件採取行動

SELECT c.country_id, c.country_name, IFNULL(MAX(b.bid_amount),0) 
FROM countries AS c 
LEFT JOIN items_bids AS b 
ON c.country_id = b.bid_country_id 
LEFT JOIN advertisers AS a 
ON a.advertiser_id = b.bid_advertiser_id 
LEFT JOIN items AS n 
ON b.bid_item_id = n.item_id 
GROUP BY c.country_name 

我敢肯定，這應該是一個不同的結果，因爲如果我把這些條件放在WHERE子句中，而不是ON子句，它確實有效。

問題是，我正在使用LEFT JOIN，因爲我不想從結果中消除任何國家。

Answer 1

您的on-條件不按預期工作，因爲您使用left join連接表，然後從不使用它們，因此它們不起作用。您只使用max(b.bid_amount)和countries，因此只有在b和countries的條件纔會生效。

例如，如果某些行的advertisers存在即滿足您的on -condition，它們結合，但對b.bid_amount沒有影響，因爲你不使用它們（他們會對例如count(*)或sum(b.bid_amount)效果雖然）。如果advertisers沒有行，則由於left join，它們對b.bid_amount也沒有影響。所以你的條件是什麼並不重要。

你的第一個代碼就相當於

SELECT c.country_id, c.country_name, IFNULL(MAX(b.bid_amount),0) 
FROM countries AS c 
LEFT JOIN items_bids AS b 
ON c.country_id = b.bid_country_id 
AND b.bid_removed = 0 
GROUP BY c.country_name 

和你的第二個代碼相當於

SELECT c.country_id, c.country_name, IFNULL(MAX(b.bid_amount),0) 
FROM countries AS c 
LEFT JOIN items_bids AS b 
ON c.country_id = b.bid_country_id 
GROUP BY c.country_name 

，它可能不是奇怪，他們給出相同的結果（取決於 您的數據和有多少行b.bid_removed = 0。嘗試b.bid_removed = 1，你可能會看到一個效果，這是唯一相關的條件）。

如果你把你的條件，例如條款中的AND a.advertiser_balance > 0，mysql只會選擇a.advertiser_balance不爲空的行！所以你的left join將作爲join，並不奇怪，這可能會有不同的結果。

由於您沒有描述您實際想要達到的目標，我會大膽猜測並刪除一些左連接。您將獲得所有國家，並且您的所有on條件都可以使用。

嘗試

SELECT c.country_id, c.country_name, IFNULL(MAX(b.bid_amount),0) 
FROM countries AS c 
LEFT JOIN items_bids AS b 
ON c.country_id = b.bid_country_id 
AND b.bid_removed = 0 
JOIN advertisers AS a 
ON a.advertiser_id = b.bid_advertiser_id 
AND a.advertiser_balance > 0 
AND a.advertiser_review = 0 
JOIN items AS n 
ON b.bid_item_id = n.item_id 
AND n.item_approved = 1 
AND n.item_deleted = 0 
AND n.item_paused = 0 
GROUP BY c.country_name