左连接条件不影响结果

Question

我试图用一些条件创建一个左连接查询，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 

它具有相同的结果，因为这：它没有被任何的ON子句条件采取行动

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，这应该是一个不同的结果，因为如果我把这些条件放在WHERE子句中，而不是ON子句，它确实有效。

问题是，我正在使用LEFT JOIN，因为我不想从结果中消除任何国家。

Answer 1

您的on-条件不按预期工作，因为您使用left join连接表，然后从不使用它们，因此它们不起作用。您只使用max(b.bid_amount)和countries，因此只有在b和countries的条件才会生效。

例如，如果某些行的advertisers存在即满足您的on -condition，它们结合，但对b.bid_amount没有影响，因为你不使用它们（他们会对例如count(*)或sum(b.bid_amount)效果虽然）。如果advertisers没有行，则由于left join，它们对b.bid_amount也没有影响。所以你的条件是什么并不重要。

你的第一个代码就相当于

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 

和你的第二个代码相当于

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，他们给出相同的结果（取决于 您的数据和有多少行b.bid_removed = 0。尝试b.bid_removed = 1，你可能会看到一个效果，这是唯一相关的条件）。

如果你把你的条件，例如条款中的AND a.advertiser_balance > 0，mysql只会选择a.advertiser_balance不为空的行！所以你的left join将作为join，并不奇怪，这可能会有不同的结果。

由于您没有描述您实际想要达到的目标，我会大胆猜测并删除一些左连接。您将获得所有国家，并且您的所有on条件都可以使用。

尝试

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