SQL多次将一个表连接到另一个表? (将产品映射到类别) [英] SQL to join one table to another table multiple times? (Mapping products to categories)

问题描述

假设我有一个Product，Category和Product_To_Category表.一个产品可以在多个类别中.

Let's say I have a Product, Category, and Product_To_Category table. A Product can be in multiple categories.

    Product                     Category        Product_to_category
    ID   |   NAME               ID  | Name      Prod_id | Cat_id
    =====================       ============    ===================
        1| Rose                    1| Flowers          1| 1
        2| Chocolate Bar           2| Food             2| 2
        3| Chocolate Flower                            3| 1
                                                       3| 2

我想要一个SQL查询，它给我这样的结果

I would like an SQL query which gives me a result such as

    ProductName      | Category_1 | Category_2 | Category_3
    =======================================================
    Rose             | Flowers    |            |
    Chocolate Flower | Flowers    | Food       |

等

我能够做到这一点的最好方法是将一堆查询结合在一起.一个查询，用于查询给定产品的每个预期类别数.

The best way I've been able to get this is to union a bunch of queries together; one query for every expected number of categories for a given product.

select p.name, cat1.name, cat2.name
from
  product p, 
  (select * from category c, producttocategory pc where pc.category_id = c.id) cat1,
  (select * from category c, producttocategory pc where pc.category_id = c.id) cat2
where p.id = cat1.id 
  and p.id = cat2.id
  and cat1.id != cat2.id
union all
select p.name, cat1.name, null
from
  product p, 
  (select * from category c, producttocategory pc where pc.category_id = c.id) cat1
where p.id = cat1.id 
  and not exists (select 1 from producttocategory pc where pc.product_id = p.id and pc.category_id != cat1.id)

这有几个问题.

• 首先，我必须为每个预期类别重复此联合；如果一个产品可以归为8个类别，则我需要8个查询.
• 第二，类别没有统一地放在同一列中.例如，有时某个产品可能带有食物，鲜花"，而另一个时候可能带有花朵，食品".

有人知道更好的方法吗?另外，此技术是否有技术名称?

Does anyone know of a better way to do this? Also, does this technique have a technical name?

推荐答案

我不知道您使用的是哪种RDBMS，但是在MySQL中，您可以使用GROUP_CONCAT:

I don't know what RDBMS you're using, but in MySQL you can use GROUP_CONCAT:

SELECT
  p.name,
  GROUP_CONCAT(c.name SEPARATOR ', ') AS categories
FROM
  product p
  JOIN product_to_category pc ON p.id = pc.product_id
  JOIN category c ON c.id = pc.category_id
GROUP BY
  p.name
ORDER BY
  p.name,
  c.name

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