mysql - 查询每个商品的购买数量列表

Question

我的 MySQL 不如其他的那么敏锐，在过去的一个小时里我一直在努力解决这个问题。也许你能给我指出正确的方向。我有一份产品 list ，我正试图从中获取一份报告。这是我的:

-- Product list: tbl_products
id       product_name
------  ----------------
1        Hamburger
2        Cheeseburger
3        Burger Balls


-- List of orders: tbl_orders
id       created
------  -----------------------
23       2013-09-04 00:00:00
45       2013-09-05 00:00:00
67       2013-09-09 00:00:00


-- Products bought per order: tbl_product_orders
order_id    product_id      quantity
----------  --------------  --------
23          1                4
23          2                6
45          1                1
67          3                2

这就是我试图通过一个查询实现的目标:

-- How do I get this
order_id    Hamburger     Cheeseburger    Burger Balls
----------  ------------- --------------- --------------
23           4             6               0
45           1             0               0
67           0             0               2

有人知道怎么做吗？

编辑: 我稍后会发布我之前的脚本。我现在只需要跑去上课。 :)

Answer 1

你可以用动态SQL来做

SET @sql = NULL;

SELECT GROUP_CONCAT(CONCAT('SUM(CASE WHEN product_id = ', id,
                           ' THEN quantity ELSE 0 END) `', product_name, '`'))
  INTO @sql
  FROM tbl_products;

SET @sql = CONCAT('SELECT order_id,', @sql,
                  '  FROM tbl_product_orders
                    GROUP BY order_id');

PREPARE stmt FROM @sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;

输出:

| ORDER_ID | HAMBURGER | CHEESEBURGER | BURGER BALLS |
|----------|-----------|--------------|--------------|
|       23 |         4 |            6 |            0 |
|       45 |         1 |            0 |            0 |
|       67 |         0 |            0 |            2 |

Here is SQLFiddle demo

---

To simplify things on calling end you can wrap it into a stored procedure

DELIMITER $$
CREATE PROCEDURE sp_report()
BEGIN
  SET @sql = NULL;

  SELECT GROUP_CONCAT(CONCAT('SUM(CASE WHEN product_id = ', id,
                             ' THEN quantity ELSE 0 END) `', product_name, '`'))
    INTO @sql
    FROM tbl_products;

  SET @sql = CONCAT('SELECT order_id,', @sql,
                    '  FROM tbl_product_orders
                      GROUP BY order_id');

  PREPARE stmt FROM @sql;
  EXECUTE stmt;
  DEALLOCATE PREPARE stmt;
END$$
DELIMITER ;

这是 SQLFiddle 演示