将连接表的结果限制为一行

Question

这是一个简化的表结构：

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

TABLE product_photos (
 product_photo_id (primary key, 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（根据photo_order）为默认照片。

现在，我只需要产品详细信息页面上的所有照片，但在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，并为每张照片重复产品信息：

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（或者是否可能）

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：我在下面的答案的帮助下找到了解决方案，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

Answer 1

SELECT p.*, ph.*
FROM products AS p
INNER JOIN product_photos AS ph
    ON p.product_id = ph.product_id
LEFT JOIN product_photos AS ph2
    ON p.product_id = ph2.product_id
    AND ph2.photo_order < ph.photo_order
WHERE ph2.photo_order IS NULL
ORDER BY p.product_title ASC

注意它如何两次连接到product_photos 表。

WHERE ph2.photo_order IS NULL

将丢弃除最低照片顺序之外的所有照片。不过，它不会保护您免受重复的product_id / photo_orders组合的影响，如果是这种情况，您可以在p.id上添加

GROUP BY

。

Answer 2

用途：

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

Answer 3

    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, ph.photo_order ASC
    GROUP BY p.product_id
    LIMIT 0,10

Answer 4

SELECT ...
  ....
GROUP BY p.product_id