Leetcode1327. 列出指定时间段内所有的下单产品（简单）

题目
表: Products

+------------------+---------+
| Column Name      | Type    |
+------------------+---------+
| product_id       | int     |
| product_name     | varchar |
| product_category | varchar |
+------------------+---------+

product_id 是该表主键。
该表包含该公司产品的数据。
表: Orders

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| product_id    | int     |
| order_date    | date    |
| unit          | int     |
+---------------+---------+

该表无主键，可能包含重复行。
product_id 是表单 Products 的外键。
unit 是在日期 order_date 内下单产品的数目。

写一个 SQL 语句，要求获取在 2020 年 2 月份下单的数量不少于 100 的产品的名字和数目。

返回结果表单的顺序无要求。

查询结果的格式如下：

Products 表:

+-------------+-----------------------+------------------+
| product_id  | product_name          | product_category |
+-------------+-----------------------+------------------+
| 1           | Leetcode Solutions    | Book             |
| 2           | Jewels of Stringology | Book             |
| 3           | HP                    | Laptop           |
| 4           | Lenovo                | Laptop           |
| 5           | Leetcode Kit          | T-shirt          |
+-------------+-----------------------+------------------+

Orders 表:

+--------------+--------------+----------+
| product_id   | order_date   | unit     |
+--------------+--------------+----------+
| 1            | 2020-02-05   | 60       |
| 1            | 2020-02-10   | 70       |
| 2            | 2020-01-18   | 30       |
| 2            | 2020-02-11   | 80       |
| 3            | 2020-02-17   | 2        |
| 3            | 2020-02-24   | 3        |
| 4            | 2020-03-01   | 20       |
| 4            | 2020-03-04   | 30       |
| 4            | 2020-03-04   | 60       |
| 5            | 2020-02-25   | 50       |
| 5            | 2020-02-27   | 50       |
| 5            | 2020-03-01   | 50       |
+--------------+--------------+----------+

Result 表:

+--------------------+---------+
| product_name       | unit    |
+--------------------+---------+
| Leetcode Solutions | 130     |
| Leetcode Kit       | 100     |
+--------------------+---------+

2020 年 2 月份下单 product_id = 1 的产品的数目总和为 (60 + 70) = 130 。
2020 年 2 月份下单 product_id = 2 的产品的数目总和为 80 。
2020 年 2 月份下单 product_id = 3 的产品的数目总和为 (2 + 3) = 5 。
2020 年 2 月份 product_id = 4 的产品并没有下单。
2020 年 2 月份下单 product_id = 5 的产品的数目总和为 (50 + 50) = 100 。

解答
先选出 2020 年 2 月份的订单

select *
from Orders as O
where year(O.order_date) = 2020 and month(O.order_date) = 2

对product_id进行分组 计算总unit 筛选器不少于100的情况

select O.product_id, sum(o.unit) as unit
from Orders as O
where year(O.order_date) = 2020 and month(O.order_date) = 2
group by O.product_id
having sum(O.unit) >=100

再和产品表连接即可

select P.product_name, tmp.unit
from (select O.product_id, sum(o.unit) as unit
from Orders as O
where year(O.order_date) = 2020 and month(O.order_date) = 2
group by O.product_id
having sum(O.unit) >=100) as tmp
join Products as P
on P.product_id = tmp.product_id;

这样写也是ok 的

select p.product_name,sum(o.unit) as unit
from
    products p
left join
    orders o 
on
    p.product_id=o.product_id
where
    order_date between '2020-02-01' and '2020-02-29'
group by
    p.product_name
having
    sum(o.unit)>=100
order by
    sum(o.unit)