SQL to join one table to another table multiple times? (Mapping products to categories)

Question

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

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       |

etc.

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)

There are several problems with this.

• First, I have to repeat this union for each expected category; if a product can be in 8 categories I'd need 8 queries.
• Second, the categories are not uniformly put into the same columns. For example, sometimes a product might have 'Food, Flowers' and another time 'Flowers, Food'.

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

Answer 1

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