题175. 组合两个表 (左连接)
思路:左连接LEFT JOIN , 右表是可能地址为空的address表。
SELECT p.FirstName,p.LastName,a.City,a.State FROM Person AS p LEFT JOIN Address AS a ON p.PersonId=a.PersonId
题183. 从不订购的客户:(左连接的过滤)
思路:通过左连接显示所有客户的订单,没有订购任何东西的orders表Id为null
SELECT NAME AS Customers FROM Customers AS c LEFT JOIN Orders AS o ON c.Id=o.CustomerId WHERE o.Id IS null;