LeetCode：Database 96.净现值查询

Posted 2021-06-29 Xiao Miao

要求：写一个 SQL, 找到 Queries 表中每一次查询的净现值。

表: NPV的结构

+---------------+---------+
| 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的结构

+---------------+---------+
| 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.

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   |
+------+--------+

Result Table：

+------+--------+--------+
| 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.

SQL语句：

select a.id,a.year,ifnull(b.npv,0) as npv
from queries a 
left join npv b 
on a.id=b.id and a.year=b.year;