如何計算SQL中項目的新外觀？

Question

比方說，我有一個訂單表與字段： user_id，created_at。

我想創建一個查詢，顯示每天出現多少個新用戶ID。

這說明多少訂單有每天：

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