如何计算SQL中项目的新外观？

Question

假设我有一个包含字段的Orders表： user_id，created_at。

我想创建一个查询，显示每天有多少NEW user_ids出现。

这显示每天有多少订单：

SELECT
  count(1), TO_CHAR(created_at, 'YYYY-MM-DD') as day
FROM
  orders
GROUP BY
  day
ORDER BY
  day DESC

Answer 1

SELECT count(1),
       TO_CHAR(FirstOrder, 'YYYY-MM-DD') AS day
FROM   (SELECT MIN(created_at) AS FirstOrder,
               user_id
        FROM   orders
        GROUP  BY user_id) first_orders
GROUP  BY TO_CHAR(FirstOrder, 'YYYY-MM-DD')
ORDER  BY day DESC