一个查询中具有不同条件的多个SELECT语句

Question

我有以下表格：

'价格'

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

“产品”

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

此处的连接在Products.id和Prices.products_id之间。

我正在运行一个脚本，该脚本可以获取URL，从特定网页中抓取价格，然后每小时更新一次“价格”表。

我想显示以下内容，最好是使用一个数据库查询：

• products.product_title
• products.product_image
• prices.price（当前价格，即每个产品ID的最后一个插入）
• 今天的平均价格和昨天的平均价格之间的百分比差异
• 当月平均价格与上个月平均价格之间的百分比差异

我可以单独执行这些操作，但是无法将它们合并到一个查询中。

为了清楚起见，我想一次抓取特定类别中的所有产品-考虑到我一直在这样做：

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

我直接在编写SQL，特别是使用MySQLdb Python库。

Answer 1

第一个子选择的未经测试的示例（在此模板上进行其他子选择）：

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

根据@Cosmin的建议的最终摘要...

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;

不确定100％如何执行查询中的百分比差异计算，但我只是使用Python做到这一点以保持查询整洁。