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What is the difference between these two Django Q-based queries:
class Item(models.Model):
    name = models.CharField(max_length=10)

class Relation(models.Model):
    item = models.ForeignKey(Item)
    weight = models.IntegerField()

some = Q(relation__x__gt=3)
others = Q(relation__x=7)

first = Item.objects.filter(some, ~others)

second = Item.objects.filter(some).exclude(others)

SELECT "item_item"."id", "item_item"."name"
FROM "item_item"
INNER JOIN "item_relation"
ON ("item_item"."id" = "item_relation"."item_id")
WHERE ("item_relation"."weight" > 3
  AND NOT ("item_item"."id" IN
    (SELECT U1."item_id"
     FROM "item_relation" U1
     WHERE (U1."weight" = 7  AND U1."item_id" IS NOT NULL))))

SELECT "item_item"."id", "item_item"."name"
FROM "item_item"
INNER JOIN "item_relation"
ON ("item_item"."id" = "item_relation"."item_id")
INNER JOIN "item_relation" T3
ON ("item_item"."id" = T3."item_id")
WHERE ("item_relation"."weight" > 3  AND NOT (T3."weight" = 7 ))