SQL将一个表连接到另一个表多次？(将产品映射到类别)

Question

比方说,我有一个Product,Category和Product_To_Category表.产品可以分为多个类别.

    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,它给我一个结果,如

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

等等

我能够得到这个的最好方法是将一堆查询结合在一起; 对给定产品的每个预期数量的类别进行一次查询.

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,我必须为每个预期的类别重复这个联盟; 如果一个产品可以分为8个类别,我需要8个查询.
• 其次,类别不是统一放入相同的列.例如,有时产品可能有'食物,花',另一种时候'花,食物'.

有谁知道更好的方法吗？此外,这种技术有技术名称吗？

Answer 1

我不知道你正在使用什么RDBMS,但在MySQL中你可以使用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