文巾解题 175. 组合两个表

Posted 2021-07-16 刘文巾

1 题目描述

2 解题思路

2.1 方法1：不用JOIN

SELECT
    FirstName, 
    LastName, 
    (SELECT City 
     FROM Address a 
     WHERE a.PersonId=p.PersonId) as City, 
    (SELECT State 
     FROM Address a 
     WHERE a.PersonId=p.PersonId) as State  
FROM Person p

如果 a.PersonId=p.PersonId不成立，那么select返回的就是NULL，也满足语义条件。

2.2 方法2 外连接

外连接知识可以看数据库笔记——数据模型_刘文巾的博客-CSDN博客 3.10节

SELECT FirstName, LastName, City, State
FROM Person left join Address
ON Person.PersonId = Address.PersonId