Leetcode1421. 净现值查询（中等）

题目
表: NPV

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| id            | int     |
| year          | int     |
| npv           | int     |
+---------------+---------+

(id, year) 是该表主键.
该表有每一笔存货的年份, id 和对应净现值的信息.

表: Queries

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| id            | int     |
| year          | int     |
+---------------+---------+

(id, year) 是该表主键.
该表有每一次查询所对应存货的 id 和年份的信息.

写一个 SQL, 找到 Queries 表中每一次查询的净现值.

结果表没有顺序要求.

查询结果的格式如下所示:

NPV 表:

+------+--------+--------+
| id   | year   | npv    |
+------+--------+--------+
| 1    | 2018   | 100    |
| 7    | 2020   | 30     |
| 13   | 2019   | 40     |
| 1    | 2019   | 113    |
| 2    | 2008   | 121    |
| 3    | 2009   | 12     |
| 11   | 2020   | 99     |
| 7    | 2019   | 0      |
+------+--------+--------+

Queries 表:

+------+--------+
| id   | year   |
+------+--------+
| 1    | 2019   |
| 2    | 2008   |
| 3    | 2009   |
| 7    | 2018   |
| 7    | 2019   |
| 7    | 2020   |
| 13   | 2019   |
+------+--------+

结果表:

+------+--------+--------+
| id   | year   | npv    |
+------+--------+--------+
| 1    | 2019   | 113    |
| 2    | 2008   | 121    |
| 3    | 2009   | 12     |
| 7    | 2018   | 0      |
| 7    | 2019   | 0      |
| 7    | 2020   | 30     |
| 13   | 2019   | 40     |
+------+--------+--------+

(7, 2018)的净现值不在 NPV 表中, 我们把它看作是 0.
所有其它查询的净现值都能在 NPV 表中找到.

解答
很简单 左连接即可

select Q.id, Q.year, ifnull(N.npv, 0) as npv
from Queries as Q
left join NPV as N
on Q.id = N.id and Q.year = N.year