Approximate statistical functions

-- Complete the query to approximate the number of customers that have ordered each product

SELECT
  items.product_id,
  -- Add the correct syntax for the approximate distinct count
  ___ AS estimated_unique_customers
FROM ecommerce.ecomm_orders o, UNNEST(o.order_items) items
JOIN ecommerce.ecomm_order_details od USING (order_id)
GROUP BY items.product_id;