如何在MySQL中使用准备好的语句创建动态列？

问题描述

我有4个彼此相关的表。

CREATE TABLE `location` (
  `id` bigint(20) UNSIGNED NOT NULL AUTO_INCREMENT,`name` varchar(255) COLLATE utf8mb4_unicode_ci NOT NULL,PRIMARY KEY (id)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;

INSERT INTO `location` (`id`,`name`) VALUES
(1,'Dallas'),(2,'Boston'),(3,'Houston');

CREATE TABLE `item` (
 `id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,`brand` varchar(255) COLLATE utf8mb4_unicode_ci DEFAULT NULL,PRIMARY KEY (id)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;

INSERT INTO `item` (`id`,`brand`) VALUES
(1,'Nissan Almera M/T 2009-2015'),'Toyota Corolla A/T 2005-2012'),'Nissan Terra A/T 2010-2017'),(4,'Suzuki Esteem M/T 1980-1990'),(5,'Toyota Fortuner A/T 2014-2020');

CREATE TABLE `item_in` (
  `id` bigint(20) UNSIGNED NOT NULL AUTO_INCREMENT,`location_id` bigint(20) UNSIGNED NOT NULL,`item_id` bigint(20) UNSIGNED NOT NULL,`quantity` int(11) DEFAULT NULL,PRIMARY KEY (id)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;

INSERT INTO `item_in` (`id`,`location_id`,`item_id`,`quantity`) VALUES
(1,1,1000),2,500),200),300),3,(6,800),(7,5,(8,4,400);

CREATE TABLE `item_out` (
  `id` bigint(20) UNSIGNED NOT NULL AUTO_INCREMENT,PRIMARY KEY (id)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;

INSERT INTO `item_out` (`id`,20),25),10),15),50);

使用动态SQL，我能够根据它们的位置和项目（item_in数量减去item_out数量）获得每个项目的单个剩余数量，并将位置名称作为列。（请参见下面的代码）：

SET @sql = NULL,@sql1 = NULL,@sql2 = NULL;

SELECT GROUP_CONCAT( DISTINCT
          CONCAT('SUM(CASE WHEN `location_id` = ''',''' THEN quantity END) AS ',`name`))
          INTO @sql1
          FROM item_in
          JOIN location on location.id = item_in.location_id;
        
SELECT GROUP_CONCAT( DISTINCT
          CONCAT('SUM(CASE WHEN `location_id` = ''',`name`))
          INTO @sql2
          FROM item_out
          JOIN location on location.id = item_out.location_id;
          
SET @sql = CONCAT('SELECT item.brand AS Item,IFNULL(item_in.Dallas,0) - IFNULL(item_out.Dallas,0) AS Dallas,IFNULL(item_in.Boston,0) - IFNULL(item_out.Boston,0) AS Boston,IFNULL(item_in.Houston,0) - IFNULL(item_out.Houston,0) AS Houston FROM item LEFT JOIN (SELECT item_in.item_id,',@sql1,' FROM item_in
                    GROUP BY item_in.item_id) AS item_in ON item.id = item_in.item_id LEFT JOIN (SELECT item_out.item_id,@sql2,' FROM item_out
                    GROUP BY item_out.item_id) AS item_out ON item.id = item_out.item_id');
                    
PREPARE stmt FROM @sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;

结果：

Item                           | Dallas | Boston | Houston
Nissan Almera M/T 2009-2015          775        0         0                           
Toyota Corolla A/T 2005-2012         480      425         0
Nissan Terra A/T 2010-2017           800        0       275
Suzuki Esteem M/T 1980-1990            0        0       385
Toyota Fortuner A/T 2014-2020          0        0       290

我的问题是，如何更改代码，以便动态显示位置名称列，而不是在查询中手动对其进行硬编码，因为用户可以随时添加新位置？如果有人可以看一下我的代码，我将非常感谢您的帮助。我唯一遇到的麻烦是如何不对这些行进行硬编码并动态地进行处理：

IFNULL(item_in.Dallas,0) AS Houston

解决方法

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