数据库左连接 a left join b on

左连接查询

保留左边主表的所有行（即使在右表没有匹配的行），右表输出满足 on 条件的行，不满足的输出null

 

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示例：组合两个表 - 力扣

表1: Person

+--------------+-----------+
| 列名          |      类型 |
+--------------+-----------+
| PersonId   |         int |
| FirstName | varchar |
| LastName | varchar |
+-------------+------------+
PersonId 是上表主键

表2: Address

+-------------+------------+
| 列名          | 类型　  |
+-------------+------------+
| AddressId | int         |
| PersonId   | int         |
| City           | varchar |
| State         | varchar |
+-------------+------------+
AddressId 是上表主键
 

编写一个 SQL 查询，满足条件：无论 person 是否有地址信息，都需要基于上述两表提供 person 的以下信息：

 FirstName, LastName, City, State 

# from A
# left join B
# on 

select A.FirstName, A.LastName, B.City, B.State
from Person as A
left join Address as B
on A.PersonId=B.PersonId
(170ms)


# 本题可优化思路：Address表可能重复保存某条地址信息，因此Address表先用distinct去重

select A.FirstName, A.LastName, B.City, B.State
from Person as A
left join (select distinct PersonId, City, State from Address) as B
on A.PersonId=B.PersonId
(150ms)

输入：
{"headers": {"Person": ["PersonId", "LastName", "FirstName"], "Address": ["AddressId", "PersonId", "City", "State"]}, "rows": {"Person": [[1, "Wang", "Allen"]], "Address": [[1, 2, "New York City", "New York"]]}}

输出：
{"headers":["FirstName","LastName","City","State"],"values":[["Allen","Wang",null,null]]}