MySQL选择具有唯一属性值的一行

Question

我正在建立一个库存系统，并决定将每种产品的余额（每次更新）存储到下表中：

+------------+--------------+---------+------+
| Product_id | Warehouse_id | Balance | Date |
+------------+--------------+---------+------+

例：

工作人员将10件商品添加到Warehouse_id 5中的product_id 123

+------------+--------------+---------+-------------+
| Product_id | Warehouse_id | Balance | Date        |
+------------+--------------+---------+-------------+
| 123        | 5            | 10      | 2013-09-16  |
+------------+--------------+---------+-------------+

然后，工作人员将3件商品添加到Warehouse_id 5中的产品234中，并将5件商品添加到Warehouse_id 5中的123中，

+------------+--------------+---------+-------------+
| Product_id | Warehouse_id | Balance | Date        |
+------------+--------------+---------+-------------+
| 123        | 5            | 10      | 2013-09-16  |
| 234        | 5            | 3       | 2013-09-18  |
| 123        | 5            | 15      | 2013-09-21  |
+------------+--------------+---------+-------------+

*注意日期栏

现在让我再添加几行

+------------+--------------+---------+-------------+
| Product_id | Warehouse_id | Balance | Date        |
+------------+--------------+---------+-------------+
| 123        | 5            | 10      | 2013-09-16  |
| 234        | 5            | 3       | 2013-09-18  |
| 123        | 5            | 15      | 2013-09-21  |
| 123        | 5            | 18      | 2013-09-24  |
| 234        | 5            | 10      | 2013-09-26  |
| 123        | 5            | 22      | 2013-09-29  |
+------------+--------------+---------+-------------+

---

我如何运行查询以获取截至2013年9月25日所有产品的余额？

这意味着我需要以下结果：

+------------+--------------+---------+-------------+
| Product_id | Warehouse_id | Balance | Date        |
+------------+--------------+---------+-------------+
| 234        | 5            | 3       | 2013-09-18  |
| 123        | 5            | 18      | 2013-09-24  |
+------------+--------------+---------+-------------+

简而言之，我需要每个product_id最新的行（按日期）。

任何帮助将不胜感激！

Answer 1

假设每个仓库都保持着产品的余额，您可以这样做

SELECT t.product_id, t.warehouse_id, t.balance, t.date
  FROM table1 t JOIN 
(
  SELECT warehouse_id, product_id, MAX(date) date
    FROM table1
   WHERE date <= '2013-09-25'
   GROUP BY warehouse_id, product_id
) q
    ON t.warehouse_id = q.warehouse_id
   AND t.product_id = q.product_id
   AND t.date = q.date

输出：

| PRODUCT_ID | WAREHOUSE_ID | BALANCE |       DATE |
|------------|--------------|---------|------------|
|        234 |            5 |       3 | 2013-09-18 |
|        123 |            5 |      18 | 2013-09-24 |

这是SQLFiddle演示

Answer 2

SELECT * 
FROM TABLE 
WHERE (PRODUCT_ID, DATE) IN
     (SELECT PRODUCT_ID, MAX(DATE) FROM TABLE
      WHERE DATE <= '2013-09-25'     
       GROUP BY PRODUCT_ID )

Answer 3

查询：

SQLFIDDLE示例

  SELECT *
  FROM table1 t 
  WHERE t.`Date` = (SELECT MAX(t2.`Date`)
                  FROM Table1 t2
                  WHERE t2.`Date` <= '2013-09-25'
                  AND t2.product_id = t.product_id)