SQL子查詢語法 產品銷售分析III Leetcode #1070

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題目敘述

題目會給我們兩張資料表，第一張是Sales，第二張是Product。

第一張是Sales表格，裡面分別有sale_id、 product_id、year、quantity、price等欄位。其中(sale_id、 product_id)做為複合主鍵Primary key

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.
product_id is a foreign key (reference column) to Product table.
Each row of this table shows a sale on the product product_id in a certain year.
Note that the price is per unit.

第二張是Product表格，裡面分別有product_id 、 product_name 等欄位。其中 product_id做為主鍵Primary key

Table: Product

+--------------+---------+
| Column Name  | Type    |
+--------------+---------+
| product_id   | int     |
| product_name | varchar |
+--------------+---------+
product_id is the primary key (column with unique values) of this table.
Each row of this table indicates the product name of each product.

要求我們，以第一次銷售出去的年份為基準，
列出銷售出去的 product_id, year, quantity, 和 price。

輸出的順序不拘。

詳細的題目可在這裡看到

測試範例

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

Product table:
+------------+--------------+
| product_id | product_name |
+------------+--------------+
| 100        | Nokia        |
| 200        | Apple        |
| 300        | Samsung      |
+------------+--------------+

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

演算法

考點主要落在抓取"第一次銷售出去的年份"，這邊可以使用subquery，SQL查詢裡面再包著另一個(SQL)子查詢的技巧，找出來。

一般常見的通則樣貌:

SELECT ...欄位 FROM ...表格

WHERE ...欄位 ... 做條件判斷(

包著另一個SQL 查詢語法

)

Subquery教學

每個產品的第一次銷售出去的年分，用SQL語法來寫，其實就是

　SELECT product_id, MIN(year) as year
　FROM Sales
　GROUP BY product_id

其中 MIN(year)就對應到題目所要求的"第一次銷售出去的年分"。

GROUP BY product_id 則是由商品編號product_id去做分群。

程式碼

# Write your MySQL query statement below
SELECT product_id, year AS first_year, quantity, price
FROM Sales
# Select the product id, year, quantity, and price for the first year of every product sold.
WHERE (product_id, year) IN (
　SELECT product_id, MIN(year) as year
　FROM Sales
　GROUP BY product_id
);

關鍵知識點

掌握子查詢的語法與架構

SELECT ...欄位 FROM ...表格

WHERE ...欄位 ... 做條件判斷(

包著另一個SQL 查詢語法

)

Reference:

[1] MySQL by subquery on composite fields - Product Sales Analysis III - LeetCode