将联接表中的结果限制为一行 [英] Limit results from joined table to one row

问题描述

这是一个简化的表结构:

Here is a simplified table structure:

TABLE products (
 product_id INT (primary key, auto_increment),
 category_id INT,
 product_title VARCHAR,
 etc
);

TABLE product_photos (
 product_photo_id (primary key, auto_increment),
 product_id INT,
 photo_href VARCHAR,
 photo_order INT
);

一个产品可以有多张照片，每个产品的第一张产品照片(基于photo_order)是默认照片.

A product can have multiple photos, the first product photo for each product (based on the photo_order) is the default photo.

现在，我只需要产品详细信息页面上的所有照片，但是在列出多个产品的页面(例如产品目录页面)上，我只想显示默认照片.

Now, I only need all of the photos on the product details page, but on pages where I am listing multiple products, for example a product directory page, I only want to display the default photo.

所以我要尝试的是查询产品列表，包括每个产品的默认照片.

So what I am trying to do, is query a list of products including the default photo for each product.

这显然行不通，它将返回所有照片，其中每张照片都有重复的产品信息:

This obviously doesn't work, it will return all photos with the product info duplicated for each photo:

SELECT p.*, ph.*
FROM products AS p
LEFT JOIN product_photos AS ph
ON p.product_id=ph.product_id
ORDER BY p.product_title ASC

我需要弄清楚该如何做，但是我不知道语法(或者是否可能)

I need to figure out how to do something like this, but I don't know the syntax (or if it is possible)

SELECT p.*, ph.*
FROM products AS p
LEFT JOIN product_photos AS ph
    ON p.product_id=ph.product_id  **ORDER BY ph.photo_order ASC LIMIT 1**
ORDER BY p.product_title ASC

我从下面的答案中找到了解决方案，谢谢大家！

SELECT p.*, ph.*
FROM products AS p
LEFT JOIN product_photos AS ph 
    ON p.product_id=ph.product_id
    AND ph.photo_order =
    (
        SELECT MIN(z.photo_order)
        FROM product_photos AS z
        WHERE z.product_id=p.product_id
    )
GROUP BY p.product_id
ORDER BY p.product_title ASC

推荐答案

使用:

SELECT p.*,
       pp.*
  FROM PRODUCTS p
  JOIN PRODUCT_PHOTOS pp ON pp.product_id = p.product_id
  JOIN (SELECT x.product_id,
               MIN(x.photo_order) AS default_photo
          FROM PRODUCT_PHOTOS x
      GROUP BY x.product_id) y ON y.product_id = pp.product_id
                              AND y.default_photo  = pp.photo_order

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