Question

我有一个查询，请加入一些表格以获取产品列表，包括价格，图片，国家等。

“product_images”表可以包含零个或多个图像，但只应返回默认图像。我的问题是，无论如何，即使该特定产品的“product_images”表中没有图像，查询也应返回产品的结果。

第一个查询示例将为每个产品返回一行，但只返回一个随机图像：

SELECT `cp`.`category_id`, `p`.`id`, `p`.`master`, `p`.`status`, `p`.`sortorder`, `p`.`sku`, `p`.`stock`, `pd`.`name`, `pd`.`short_description`, `pd`.`description`, `pd`.`slug`, `pi`.`image`, `pi`.`path`
FROM `categories_products` AS `cp`
JOIN `products` AS `p` ON (`cp`.`product_id` = `p`.`id`)
JOIN `product_descriptions` AS `pd`
ON (`pd`.`product_id` = `p`.`id`)
LEFT JOIN `product_images` AS `pi`
ON (`pi`.`product_id` = `p`.`id`)
WHERE `cp`.`category_id` = 34
AND `pd`.`locale_id` = 1
AND `p`.`master` = '0'
AND `p`.`status` = '1'
AND `p`.`accessible` = '1'
AND `pd`.`status` = '1'
GROUP BY `p`.`id`
ORDER BY `p`.`sortorder`

以下查询将返回默认图像。但是，如果“product_images”中没有图像，则不会检索该产品的行。唯一的区别在于此部分：“AND pi。preset = 1”

SELECT `cp`.`category_id`, `p`.`id`, `p`.`master`, `p`.`status`, `p`.`sortorder`, `p`.`sku`, `p`.`stock`, `pd`.`name`, `pd`.`short_description`, `pd`.`description`, `pd`.`slug`, `pi`.`image`, `pi`.`path`
FROM `categories_products` AS `cp`
JOIN `products` AS `p` ON (`cp`.`product_id` = `p`.`id`)
JOIN `product_descriptions` AS `pd`
ON (`pd`.`product_id` = `p`.`id`)
LEFT JOIN `product_images` AS `pi`
ON (`pi`.`product_id` = `p`.`id`)
WHERE `cp`.`category_id` = 34
AND `pi`.`preset` = 1
AND `pd`.`locale_id` = 1
AND `p`.`master` = '0'
AND `p`.`status` = '1'
AND `p`.`accessible` = '1'
AND `pd`.`status` = '1'
GROUP BY `p`.`id`
ORDER BY `p`.`sortorder`

Answer 1

 `pi`.`preset` = 1

WHERE中的

会破坏您的目标（您的LEFT JOIN的行为类似于INNER）。将其移至ON：

LEFT JOIN `product_images` AS `pi`
ON (`pi`.`product_id` = `p`.`id` AND `pi`.`preset` = 1)

Answer 2

您可能更喜欢使用子查询：

LEFT JOIN (SELECT * FROM product_images WHERE preset = 1) AS pi ON ...

否则，WHERE子句将应用于整个查询，如果没有图像，WHERE pi.preset=1会将搜索集减少为零行。

MySQL LEFT JOIN不会返回所有结果

2 个答案: