Question

我正在尝试使用某些条件创建LEFT JOIN查询，但它似乎行为奇怪。任何ON子句条件都没有对它起作用：

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

它的结果与此相同：

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

我确定它应该是一个不同的结果，因为如果我将这些条件放在WHERE子句而不是ON子句中它确实有效。

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 - 条件无法按预期工作，因为您使用left join来连接表格，然后从不使用它们，因此它们无效。您只能使用max(b.bid_amount)和countries，因此只有b和countries上的条件才会生效。

例如，如果advertisers的某些行符合您的on条件，那么它们会被加入，但对b.bid_amount没有影响，因为您不会使用它们（它们）会对例如count(*)或sum(b.bid_amount)产生影响。如果advertisers没有行，则由于b.bid_amount，它们对left join也没有影响。所以你的条件并不重要。

您的第一个代码相当于

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 
AND b.bid_removed = 0
GROUP BY c.country_name

，您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

并且他们给出相同的结果也许并不奇怪（取决于您的数据和行数b.bid_removed = 0。试试b.bid_removed = 1您可能会看到效果，这是唯一相关的条件。）

如果你提出了条件，例如在AND a.advertiser_balance > 0 - 子句中where，mysql只会选择a.advertiser_balance不为空的行！因此，left join将充当join，并且这可能会产生不同的结果并不令人惊讶。

由于你没有描述你真正想要达到的目标，我会猜测并删除一些左连接。您将获得所有国家/地区，并且所有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

LEFT JOIN条件不影响结果

1 个答案: