Question

Yeah, kinda dumb title, but I found it hard to describe my problem. I have the following tables:

properties

id | name
1  | color
2  | material

options

id | property_id | name
1  | 1           | yellow
2  | 1           | blue
3  | 2           | wood
4  | 2           | stone

substances

id | name
1  | orange juice
2  | cheese

relations

id | substance_id | option_id
1  | 2            | 1
2  | 2            | 3
3  | 1            | 1

Now, I have a list of options and want to know which substances are related to all those options. (For example, which substances are yellow and made of wood?) Is this possible with one query?

I’m trying to do this in Rails.

Answer 1

(SELECT s.name FROM substances s, relations r, options o
WHERE r.substance_id = s.id and r.option_id = o.id and o.name='yellow')
INTERSECT
(SELECT s.name FROM substances s, relations r, options o
WHERE r.substance_id = s.id and r.option_id = o.id and o.name='wood')

Or

SELECT s.name FROM substances s
WHERE exists(SELECT * from relations r, options o 
      WHERE r.substance_id = s.id and r.option_id = o.id and o.name='yellow')
AND   exists(SELECT * from relations r, options o 
      WHERE r.substance_id = s.id and r.option_id = o.id and o.name='wood')

Having a single options table with the property_id to tell apart different kinds of options is not such a good idea and really makes this much harder. I’d suggest breaking out the different kinds of options into different tables

MATERIALS(id,name)
COLORS(id,name)

and using separate relations for each type of table. In this case, you wouldn’t need separate tables for each relation, since it appears that it’s a many (substance) to one (color) relation.

SUBSTANCES(id, name, material_id, color_id)

Then your query is much simpler

SELECT s.name FROM substances s, materials m, colors c
WHERE s.color_id = c.id AND m.material_id = m.id
AND m.name = 'wood'
AND c.name = 'yellow'

ActiveRecord should easily be able to handle this last query much more easily than the first two.