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             


        

Answer 1

Seb's answer put me onto the right track for a workaround. I am using Oracle and it has functions which emulate MYSQL's group_concat. Here is an example. This does not generate columns, and thus isn't as good as a pure SQL solution, but it is suitable for my current purposes.

with data as
( 
  select 
    pc.id cat,
    p.id prod, 
    row_number() over( partition by p.id order by pc.id) rn,
    count(*) over (partition by p.id) cnt
  from product_to_category pc, product p
  where pc.product_id = p.id
)
select prod, ltrim(sys_connect_by_path(cat, ','), ',') cats
  from data
 where rn = cnt
 start with rn = 1 connect by prior prod = prod and prior rn = rn - 1
 order by prod

This generates data such as

PROD | CATS
===========
284  |   12
285  |   12
286  | 9,12

I can edit the ltrim(sys_connect_by_path()) column as needed to generate whatever data I need.