sql – 将连接表的结果限制为一行

这是一个简化的表格结构：

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

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

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.

现在,我只需要在产品详细信息页面上的所有照片,但在我列出多个产品的页面上,例如产品目录页面,我只想显示默认照片.

所以我想做的是查询产品列表,包括每个产品的默认照片.

这显然不起作用,它将返回所有照片与每张照片重复的产品信息：

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

我需要弄清楚如何做这样的事情,但我不知道语法(或者如果可能的话)

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

编辑：我从下面的答案中找出了一个解决方案,谢谢！

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

解决方法

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