LeetCode 183. Customers Who Never Order

問題：哪個使用者沒有訂單

解法：兩者共同之處在於Customers的id等於Order的customerId，Orders裡面的customerId會告訴你誰沒有訂單，那將有訂單之外的使用者排除掉即可

SELECT name AS Customers
FROM Customers
WHERE Customers.id NOT IN
      (SELECT customerId FROM Orders);