SELECT *
FROM product
LEFT OUTER JOIN
(
    SELECT *
    FROM Cart
    LEFT OUTER JOIN
    cartproducts
    ON Cart.Id = cartproducts.Cart_id
    WHERE Cart.username = 'user'
)
AS CartFiltered
ON product.Id = CartFiltered.product_id

SELECT *
FROM product
LEFT OUTER JOIN
cartproducts
ON product.Id = cartproducts.Product_id
LEFT OUTER JOIN
Cart
ON
cartproducts.cart_id = cart.id
WHERE Cart.username = 'user'

Dim critPage As ICriteria = Session.CreateCriteria(GetType(Product)) _
.SetFirstResult(pageNumber * itemsPerPage).SetMaxResults(itemsPerPage) _
.CreateCriteria("Carts", "c", SqlCommand.JoinType.LeftOuterJoin) _
.SetProjection(plist) _
.SetResultTransformer(New TypedResultTransformer(Of ProductWithCartInfo)) _
.Add(Expression.Eq("c.User", username))

criteria.CreateAlias("Cart", "Cart", JoinType.LeftOuterJoin);

ICriterion cartCriterion = Restrictions.Eq("Cart.User", username);
customerCriterion = Restrictions.Or(customerCriterion, Restrictions.IsNull("Cart.User"));
criteria.Add(customerCriterion);