Question

I have three mysql tables

items
===========
id    title

items_in_categories
============================
id    item_id    category_id

categories
===========
id    title

I want to find all the items that belong to ALL the stated categories. Not any one category, but ALL categories

Eg, if I want to search all the items that belongs to category id 3 and 5

the no. of possible categories to be searched can go up to as many as 20.

Eg, I want to get all the items that belongs to category id 1, 2, 3, 4, 5, 6, …. and 20

I would like to use as simple a way as possible.

I have tried AND and a nested NOT EXISTS as stated in the mysql manual.

Nothing worked.

UPDATE:
I came up with this.

select * from 
    (select count(*) as counter, item_id from items_in_categories 
        where category_id in (3, 5) group by item_id)getall 
where counter = 2

Is there a better way?

Answer 1

I know this can be prettied up and put into one query (You could nest the category count within the having instead of as a separate query…I just think this is more readable), but here is my shot at this:

DECLARE @CategoryCount INT;

SELECT @CategoryCount = COUNT(DISTINCT id) AS CategoryCount
FROM categories
WHERE category_id IN (3,5);

SELECT *
FROM items
WHERE item_id IN 
(
    SELECT item_id
    FROM items_in_categories
    WHERE items_in_categories.category_id IN (3,5)
    GROUP BY item_id
    HAVING COUNT(DISTINCT category_id) = @CategoryCount
)