Question

这是我的问题：

SELECT DISTINCT `post_data`. * , pv.`seller_id` , pv.`islimited` , pv.`isquantity` , pv.`isslider`, `price`.`original_price` , `price`.`discount_percentage` , `timelimit`.`start_date` , `timelimit`.`expire_date` , `quantity`.`in_stock`, `currency`.`currency_symbol`, `seller`.`directory`, `post_to_cat`.`cat_id`, count(`sales`.`sales_id`) as sale FROM `post_view` AS pv
INNER JOIN `post_data` ON pv.`post_id` = `post_data`.`post_id` AND pv.`status` = 1 
INNER JOIN `price` ON pv.`post_id` = `price`.`post_id` 
INNER JOIN `currency` ON `price`.`currency_id` = `currency`.`currency_id` 
INNER JOIN `seller` ON pv.`seller_id` = `seller`.`seller_id` 
INNER JOIN `post_to_cat` ON `post_to_cat`.`cat_id` = 1 AND `post_to_cat`.`post_id` = `post_data`.`post_id` 
LEFT JOIN `timelimit` ON ( CASE WHEN pv.`islimited` = 1 THEN `timelimit`.`post_id` ELSE -1 END ) = pv.`post_id`
LEFT JOIN `quantity` ON  ( CASE WHEN pv.`isquantity` = 1 THEN `quantity`.`post_id` ELSE -1 END ) = pv.`post_id`
LEFT JOIN `sales` ON `sales`.`post_id` = pv.`post_id` AND `sales`.`status` = 1 
WHERE pv.`status` = 1
ORDER BY pv.`post_id` DESC LIMIT 1

ORDER BY DESC不起作用，它只返回表中的第一行，但我希望得到最高的post_id值行。我犯的错是什么？

Answer 1

AS @Alex在评论中说你最后得到了LIMIT 1，为了便于阅读，你应该将最后LEFT JOIN括起来。

Answer 2

正如@ McAdam331所说，我们需要数据样本和sql小提琴来调查你的查询有什么问题。但目前我有一些建议如何改进和调试您的查询。

首先，如果您想获得最大ID，我在查询中看到主表和左表的内容是post_view，因此所有其他表应该是LEFT JOIN。只有当您认为其他表格能够以某种方式过滤主表格并且订单或结果可能其他表格依赖时，才应使用INNER JOIN。但在你的情况下，我认为没有理由使用INNER JOIN。

第二点是你非常奇怪的ON条件：

LEFT JOIN `timelimit` ON ( CASE WHEN pv.`islimited` = 1 THEN `timelimit`.`post_id` ELSE -1 END ) = pv.`post_id`
LEFT JOIN `quantity` ON  ( CASE WHEN pv.`isquantity` = 1 THEN `quantity`.`post_id` ELSE -1 END ) = pv.`post_id`

我已将它们转换为另一个

CASE WHEN pv.`islimited`=1 THEN `timelimit`.`start_date` ELSE NULL END as start_date ,
CASE WHEN pv.`islimited`=1 THEN `timelimit`.`expire_date` ELSE NULL END as expire_date,
CASE WHEN pv.`isquantity`=1 THEN  `quantity`.`in_stock` ELSE NULL END as in_stock,

但我仍然不喜欢它。这对我来说似乎没用。我读CASE WHEN pv.islimited=1 THEN timelimit.start_date ELSE NULL END as start_date时没有任何意义，所以如果你不需要pv.islimited=0标记start_date？你确定吗？

我可以建议的最后一件事：尝试使用我的甚至是你的查询。但是在调试时逐步添加每个表。所以首先查询：

SELECT 
pv.`post_id`, pv.`seller_id` , pv.`islimited` , pv.`isquantity` ,
pv.`isslider`
FROM `post_view` AS pv
WHERE pv.`status` = 1 
ORDER BY pv.`post_id` DESC 
LIMIT 1

如果返回正确的post_id，请添加下一个表格：

SELECT 
pv.`post_id`, pv.`seller_id` , pv.`islimited` , pv.`isquantity` ,
pv.`isslider`, 
`post_data`. * 
FROM `post_view` AS pv
LEFT JOIN `post_data` 
ON pv.`post_id` = `post_data`.`post_id` 
WHERE pv.`status` = 1 
  AND `post_data`.`slug` = 'abc' 
ORDER BY pv.`post_id` DESC 
LIMIT 1

检查结果。并继续一步一步。

是的，需要时间。但那是调试过程。这可能是完成查询的最快方法。： - ）

SELECT `post_data`. * ,
pv.`post_id`, pv.`seller_id` , pv.`islimited` , pv.`isquantity` ,
pv.`isslider`, `price`.`original_price` , `price`.`discount_percentage` , 
CASE WHEN pv.`islimited`=1 THEN `timelimit`.`start_date` ELSE NULL END as start_date ,
CASE WHEN pv.`islimited`=1 THEN `timelimit`.`expire_date` ELSE NULL END as expire_date,
CASE WHEN pv.`isquantity`=1 THEN  `quantity`.`in_stock` ELSE NULL END as in_stock,
 `currency`.`currency_symbol`, `seller`.`directory`, `post_to_cat`.`cat_id`, count(`sales`.`sales_id`) as sale 
FROM `post_view` AS pv
LEFT JOIN `post_data` 
ON pv.`post_id` = `post_data`.`post_id` 
LEFT JOIN `price` 
ON pv.`post_id` = `price`.`post_id` 
LEFT JOIN `currency` 
ON `price`.`currency_id` = `currency`.`currency_id` 
LEFT JOIN `seller` 
ON pv.`seller_id` = `seller`.`seller_id` 
LEFT JOIN `post_to_cat` 
ON `post_to_cat`.`cat_id` = 1 
  AND `post_to_cat`.`post_id` = pv.`post_id` 
LEFT JOIN `timelimit` 
ON `timelimit`.`post_id` = pv.`post_id`
LEFT JOIN `quantity` 
ON quantity`.`post_id` = pv.`post_id`
LEFT JOIN `sales` 
ON `sales`.`post_id` = pv.`post_id` 
  AND `sales`.`status` = 1 
WHERE pv.`status` = 1 
  AND `post_data`.`slug` = 'abc' 
GROUP BY pv.`post_id` 
ORDER BY pv.`post_id` DESC 
LIMIT 1

编辑1 - 根据@ McAdam331添加的最后GROUP BY pv.post_id通知count()函数没有GROUP BY

Answer 3

我认为这里的问题主要是由于预先形成聚合（使用COUNT（））函数而没有任何group by。虽然，你似乎并不一定需要它，因为你只想对那个帖子进行计数。

如果您正在尝试收集单个帖子的所有信息，我会调整您的WHERE子句，以便只收集具有最大ID的帖子的信息。

使用子查询来获取最大的ID，而不是按ID排序并限制为1，如下所示：

...
WHERE pv.status = 1 AND post_data.slug = 'abc' AND pv.post_id = (SELECT MAX(post_id) FROM post_view);

几个连接后，order by子句行为不正确

3 个答案: