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

SELECT p.name, cat_food.name, cat_flowers.name
FROM
  product p
  left outer join  Product_to_category pc_food 
    on p.id = pc_food.Prod_id
  left outer join Category cat_food
    on pc_food.Cat_id = cat_food.id
    AND cat_food.name = 'Food'
  left outer join  Product_to_category pc_flowers
    on p.id = pc_flowers.Prod_id
  left outer join Category cat_flowers
    on pc_flowers.Cat_id = cat_flowers.id
    AND cat_flowers.Name = 'Flowers'

It only works if you know the number of possible categories, to put them into columns. That's how (standard) SQL works, the number of columns is not dynamic.