一個查詢中具有不同條件的多個SELECT語句

Question

我有以下表格：

'價格'

+------------+--------------+------+-----+---------+----------------+
| Field      | Type         | Null | Key | Default | Extra          |
+------------+--------------+------+-----+---------+----------------+
| id         | mediumint(9) | NO   | PRI | NULL    | auto_increment |
| product_id | int(11)      | YES  |     | NULL    |                |
| price      | text         | YES  |     | NULL    |                |
| date       | text         | YES  |     | NULL    |                |
| time       | text         | YES  |     | NULL    |                |
+------------+--------------+------+-----+---------+----------------+

“產品”

+-----------------+--------------+------+-----+---------+----------------+
| Field           | Type         | Null | Key | Default | Extra          |
+-----------------+--------------+------+-----+---------+----------------+
| id              | mediumint(9) | NO   | PRI | NULL    | auto_increment |
| category_id     | int(11)      | YES  |     | NULL    |                |
| product_urls    | text         | YES  |     | NULL    |                |
| product_title   | text         | YES  |     | NULL    |                |
| product_image   | text         | YES  |     | NULL    |                |
| product_content | text         | YES  |     | NULL    |                |
+-----------------+--------------+------+-----+---------+----------------+

此處的連接在Products.id和Prices.products_id之間。

我正在運行一個腳本，該腳本可以獲取URL，從特定網頁中抓取價格，然后每小時更新一次“價格”表。

我想顯示以下內容，最好是使用一個數據庫查詢：

• products.product_title
• products.product_image
• prices.price（當前價格，即每個產品ID的最后一個插入）
• 今天的平均價格和昨天的平均價格之間的百分比差異
• 當月平均價格與上個月平均價格之間的百分比差異

我可以單獨執行這些操作，但是無法將它們合並到一個查詢中。

為了清楚起見，我想一次抓取特定類別中的所有產品-考慮到我一直在這樣做：

...WHERE category_id=%s...GROUP BY product_id...

我直接在編寫SQL，特別是使用MySQLdb Python庫。

Answer 1

第一個子選擇的未經測試的示例（在此模板上進行其他子選擇）：

SELECT p.*,
     (SELECT prices.price
          FROM prices
          WHERE prices.product_id = p.id
          ORDER BY prices.id DESC
          LIMIT 1) as last_price
FROM product p
WHERE p.category_id = 4;

可能不是最有效的方法...

Answer 2

根據@Cosmin的建議的最終摘要...

SELECT p.*,
     (SELECT prices.price
          FROM prices
          WHERE prices.product_id = p.id
          ORDER BY prices.id DESC
          LIMIT 1) as last_price,

     (SELECT ROUND(AVG(prices.price), 2)
          FROM prices
          WHERE prices.product_id = p.id AND date = CURRENT_DATE()
          ORDER BY prices.id DESC
          LIMIT 1) as todays_average_price,

     (SELECT ROUND(AVG(prices.price), 2)
          FROM prices
          WHERE prices.product_id = p.id AND date = SUBDATE(CURRENT_DATE, 1)
          ORDER BY prices.id DESC
          LIMIT 1) as yesterdays_average_price,

     (SELECT ROUND(AVG(prices.price), 2)
          FROM prices
          WHERE prices.product_id=p.id AND MONTH(date)=MONTH(current_date)
          ORDER BY prices.id DESC
          LIMIT 1) as current_months_average_price,

     (SELECT ROUND(AVG(prices.price), 2)
          FROM prices
          WHERE prices.product_id=p.id AND MONTH(date)=MONTH(CURRENT_DATE - INTERVAL 1 MONTH)
          ORDER BY prices.id DESC
          LIMIT 1) as previous_months_average_price
FROM products p
WHERE p.category_id=%s;

不確定100％如何執行查詢中的百分比差異計算，但我只是使用Python做到這一點以保持查詢整潔。