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1070. Product Sales Analysis III
Medium
Topics
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Companies
SQL Schema
Pandas Schema
Table: Sales

+-------------+-------+
| Column Name | Type  |
+-------------+-------+
| sale_id     | int   |
| product_id  | int   |
| year        | int   |
| quantity    | int   |
| price       | int   |
+-------------+-------+
(sale_id, year) is the primary key (combination of columns with unique values) of this table.
Each row records a sale of a product in a given year.
A product may have multiple sales entries in the same year.
Note that the per-unit price.

Write a solution to find all sales that occurred in the first year each product was sold.

For each product_id, identify the earliest year it appears in the Sales table.

Return all sales entries for that product in that year.

Return a table with the following columns: product_id, first_year, quantity, and price.
Return the result in any order.



Example 1:

Input:
Sales table:
+---------+------------+------+----------+-------+
| sale_id | product_id | year | quantity | price |
+---------+------------+------+----------+-------+
| 1       | 100        | 2008 | 10       | 5000  |
| 2       | 100        | 2009 | 12       | 5000  |
| 7       | 200        | 2011 | 15       | 9000  |
+---------+------------+------+----------+-------+

Output:
+------------+------------+----------+-------+
| product_id | first_year | quantity | price |
+------------+------------+----------+-------+
| 100        | 2008       | 10       | 5000  |
| 200        | 2011       | 15       | 9000  |
+------------+------------+----------+-------+

理解解题步骤

第一步：找出每个产品最早销售的年份。

SELECT
    product_id,
    MIN(year) AS first_year
FROM Sales
GROUP BY product_id;

得到中间结果：

product_id

       

first_year




100

       

2008




200

       

2011

这里的 GROUP BY product_id 表示每个产品单独分组，MIN(year) 找出每组最小的年份。

第二步：把中间结果与原表连接。

ON s.product_id = f.product_id
AND s.year = f.first_year

两个条件都必须满足：

产品 ID 相同。

销售年份等于该产品最早销售的年份。

这样就能找回原表中的 quantity 和 price，而且不会遗漏同一产品在最早年份的其他销售记录。

方法二：使用 MIN() 窗口函数

如果你正在学习窗口函数，也可以这样写：

SELECT
    product_id,
    year AS first_year,
    quantity,
    price
FROM (
    SELECT
        *,
        MIN(year) OVER (
            PARTITION BY product_id
        ) AS first_year
    FROM Sales
) AS s
WHERE year = first_year;

这里：

PARTITION BY product_id：按产品分别计算。

MIN(year) OVER (...)：计算每个产品最早的年份，但保留原表的每一行。

WHERE year = first_year：只保留最早年份的销售记录。

1. SELECT
   
2.     s.product_id,
   
3.     s.year AS first_year,
   
4.     s.quantity,
   
5.     s.price
   
6. FROM Sales AS s
   
7. JOIN (
   
8.     SELECT
   
9.         product_id,
   
10.         MIN(year) AS first_year
    
11.     FROM Sales
    
12.     GROUP BY product_id
    
13. ) AS f
    
14. ON s.product_id = f.product_id
    
15. AND s.year = f.first_year;

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