item
id       category_id
 1            1
 2            1
 3            2
 4            3

property
 id            name
  1           'shape'
  2           'color'

propvalues
id          property_id      item_id    value
 1            1                1          'round'
 2            2                1          'grey'
 3            1                2          'round'
 4            2                2          'black'
 5            1                3          'square'
 6            2                3          'black'

SELECT item.id FROM item
JOIN propvalues ON item.id=propvalues.item_id
WHERE item.category_id IN (1,2) &&
propvalues.value IN ('black','grey') AND propvalues.value IN ('round')

WHERE item.category_id IN(1,2) &&
propvalues.property_id = 2 && propvalues.value IN ('black','grey') &&
propvalues.property_id = 1 && propvalues.value IN ('round')