Django many to one reverse query on the same field - sql

Question

I have a one to many relationship between two tables ...
Item - id, title
ItemAttribute - id, item_id, attribute_code(string, indexed)
where attribute_code can have values for colors, sizes, qualities, dimensions, etc,
- like codes for 'Blue', 'Black', 'White', 'S', 'L', 'XL', '250g', '400g', etc
Question: How do I query for all Items that are either ('Blue' OR 'White') AND 'XL'
I prefer using Django's ORM, but if someone could even help out with raw SQL it would do just fine. Thanks.

Answer 1

This subquery:
select item_id
from item_attribute
group by item_id
having sum(attribute_code in ('Blue', 'White')) > 0 and sum(attribute_code = 'XL') > 0
returns all the item_ids that you want.
So you can join it to the table item:
select i.*
from item i inner join (
select item_id
from item_attribute
group by item_id
having sum(attribute_code in ('Blue', 'White')) > 0 and sum(attribute_code = 'XL') > 0
) a on a.item_id = i.id
or use the operator IN:
select *
from item
where id in (
select item_id
from item_attribute
group by item_id
having sum(attribute_code in ('Blue', 'White')) > 0 and sum(attribute_code = 'XL') > 0
)
If you have really large data sets you could also try EXISTS:
select i.*
from item i
where
exists (select 1 from item_attribute where item_id = i.id and attribute_code in ('Blue', 'White'))
and
exists (select 1 from item_attribute where item_id = i.id and attribute_code = 'XL')