如何解决如何在MySQL中使用准备好的语句创建动态列？
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
通过示例，考虑以下内容，该内容使用PHP和mysqli_ API。 （我使用了过程代码，但是dbo会更有效）...
<?php
/*
DROP TABLE IF EXISTS location;
CREATE TABLE `location` (
`id` bigint(20) UNSIGNED NOT NULL AUTO_INCREMENT,`name` varchar(255) NOT NULL,PRIMARY KEY (id)
);
INSERT INTO `location` (`id`,`name`) VALUES
(1,'Dallas'),(2,'Boston'),(3,'Houston');
DROP TABLE IF EXISTS item;
CREATE TABLE `item` (
`id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,`brand` varchar(255) DEFAULT NULL,PRIMARY KEY (id)
);
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');
DROP TABLE IF EXISTS item_in;
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)
);
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);
DROP TABLE IF EXISTS item_out;
CREATE TABLE `item_out` (
`id` bigint(20) UNSIGNED NOT NULL AUTO_INCREMENT,PRIMARY KEY (id)
);
INSERT INTO `item_out` (`id`,20),25),10),15),50);
*/
require('path/to/connect.ion');
$query = "
SELECT l.name city,i.brand item,SUM(x.quantity) total
FROM
( SELECT location_id,item_id,'in' type,quantity FROM item_in
UNION ALL
SELECT location_id,'out',quantity*-1 FROM item_out
) x
JOIN location l
ON l.id = x.location_id
JOIN item i
ON i.id = x.item_id
GROUP
BY item,city
ORDER
BY city,item
";
$result = mysqli_query($db,$query);
$array = array();
while($row = mysqli_fetch_assoc($result)){
$array[] = $row;
}
foreach($array as $v){
$new_array[$v['city']][$v['item']] = $v['total'];
}
print_r($new_array);
?>
输出：
Array
(
[Boston] => Array
(
[Toyota Corolla A/T 2005-2012] => 425
)
[Dallas] => Array
(
[Nissan Almera M/T 2009-2015] => 775
[Nissan Terra A/T 2010-2017] => 800
[Toyota Corolla A/T 2005-2012] => 480
)
[Houston] => Array
(
[Nissan Terra A/T 2010-2017] => 275
[Suzuki Esteem M/T 1980-1990] => 385
[Toyota Fortuner A/T 2014-2020] => 290
)
)
或者您可以在$new_array[$v['city']][$v['item']] = $v['total'];中交换城市和物品，以获得：
Array
(
[Toyota Corolla A/T 2005-2012] => Array
(
[Boston] => 425
[Dallas] => 480
)
[Nissan Almera M/T 2009-2015] => Array
(
[Dallas] => 775
)
[Nissan Terra A/T 2010-2017] => Array
(
[Dallas] => 800
[Houston] => 275
)
[Suzuki Esteem M/T 1980-1990] => Array
(
[Houston] => 385
)
[Toyota Fortuner A/T 2014-2020] => Array
(
[Houston] => 290
)
)
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