Question

I’m trying to write query to a MYSQL database that queries for products that have a relationship with all selected categories

Products table

product_id | product_name
---------------------------------
    1      | product name one
    2      | product name two
    3      | product three

Category table

category_id | category_name
    1       | category one
    2       | category two
    3       | category three

Category_relationship table

product_id | category_id
--------------------------
    1      |    1
    1      |    2
    1      |    3
    2      |    1
    2      |    2
    3      |    3

So for example:

• category ID’s 1 and 3 are selected
• Only product_id to be returned would be ‘1’

Answer 1

Using 2 JOINs

SELECT
    p.*
FROM products p
  JOIN category_relationship r1 
    ON r1.product_id = p.product_id 
  JOIN category_relationship r2
    ON r2.product_id = p.product_id 
WHERE r1.category_id = 1
  AND r2.category_id = 3

Using GROUP BY

SELECT
    p.*
FROM products p
  JOIN category_relationship r 
    ON r.product_id = p.product_id 
WHERE r.category_id IN (1,3)
GROUP BY p.product_id 
HAVING COUNT(*) = 2             <-- number of category ids

---

It depends on your tables size and data distribution but I’d guess the first query to be faster.

But if you have lists of various sizes to check (with 3, 4, … category ids), the first query has to be built dynamically while the second can be easily adjusted.