SQL: Get Products from a category but also must be in another set of categories

Question

I am currently stuck in a situation. The scenario is this. I have products who may be associated with multiple categories. The data structure is shown below:

Answer 1

This type of problem is called relational division.

There are two common solutions:

1. First solution strings together the matching categories and compares to a fixed string:

   SELECT p2c.product_id
   FROM oc_product_to_category p2c
   GROUP BY p2c.product_id
   HAVING GROUP_CONCAT(p2c.category_id SEPARATOR ',' ORDER BY p2c.category_id) = '1,2'
   

2. Second solution does a JOIN for each required value:

   SELECT p.product_id 
   FROM oc_product p 
   INNER JOIN oc_product_to_category p2c1 
     ON (p.product_id = p2c1.product_id AND p2c1.category_id = 1) 
   INNER JOIN oc_product_to_category p2c2 
     ON (p.product_id = p2c2.product_id AND p2c2.category_id = 2) 
   

I cover these solutions in my presentation SQL Query Patterns, Optimized. I found in my tests that the join solution is much better for performance.

---

@Tom's suggestion is right, here's what that would look like in a complete query:

    SELECT p.product_id, GROUP_CONCAT(p2c3.category_id SEPARATOR ',') AS categories
    FROM oc_product p 
    INNER JOIN oc_product_to_category p2c1 
      ON (p.product_id = p2c1.product_id AND p2c1.category_id = 1) 
    INNER JOIN oc_product_to_category p2c2 
      ON (p.product_id = p2c2.product_id AND p2c2.category_id = 2) 
    INNER JOIN oc_product_to_category p2c3
      ON (p.product_id = p2c3.product_id)
    GROUP BY p.product_id;

The DISTINCT that @Tom suggests shouldn't be necessary, because your p2c table should have a UNIQUE constraint over (product_id, category_id).