动态MySQL查询/视图的交叉表
我目前有一个使用以下sql的硬编码视图：
select username ,(case user_role.role_id when 1 then true else false end) as ROLE_SUPER
,(case user_role.role_id when 2 then true else false end) as ROLE_ADMIN
,(case user_role.role_id when 3 then true else false end) as ROLE_VIEW
,(case user_role.role_id when 4 then true else false end) as ROLE_USER
,(case user_role.role_id when 5 then true else false end) as ROLE_EMAIL
from user
left outer join user_role on user.id=user_role.user_id
left outer join role on user_role.role_id = role.id;
我的问题是是否可以从角色表中的记录动态生成角色列。
回答：
您 可以 做您想做的事，但是我不确定 为什么
要这么做。获得动态列别名后，如何计划引用它们？也就是说，如果您从数据库中提取列别名，那么您将如何使用它们呢？我可能错过了您提出问题的原因。
无论如何，我假设您具有这样的结构：
CREATE TABLE `user` ( `id` int(11) NOT NULL auto_increment,
`username` varchar(255) default NULL,
PRIMARY KEY (`id`)
);
CREATE TABLE `role` (
`id` int(11) NOT NULL auto_increment,
`role` varchar(255) default NULL,
PRIMARY KEY (`id`)
);
CREATE TABLE `user_role` (
`user_id` int(11),
`role_id` int(11),
PRIMARY KEY (`user_id`, `role_id`)
);
INSERT INTO `user` (`username`) VALUES
('Bob'), ('Alice'), ('Carol'), ('Dave'), ('Eve');
INSERT INTO `role` (`role`) VALUES
('Super'), ('Admin'), ('View'), ('User'), ('Email');
INSERT INTO `user_role` VALUES
(1,1), (2,2), (3,3), (4,4), (5,5);
由此，您可以获得有关用户及其角色的信息：
SELECT username, role.id AS role_id, role.role AS role FROM user_roleJOIN user ON user.id = user_role.user_id
JOIN role ON role.id = user_role.role_id;
+----------+---------+-------+
| username | role_id | role |
+----------+---------+-------+
| Bob | 1 | Super |
| Alice | 2 | Admin |
| Carol | 3 | View |
| Dave | 4 | User |
| Eve | 5 | Email |
+----------+---------+-------+
您还可以为特定角色创建列别名：
SELECT username, (role.id = 1) AS Super FROM user_roleJOIN user ON user.id = user_role.user_id
JOIN role ON role.id = user_role.role_id;
+----------+-------+
| username | Super |
+----------+-------+
| Bob | 1 |
| Alice | 0 |
| Carol | 0 |
| Dave | 0 |
| Eve | 0 |
+----------+-------+
但是，如果我正确理解了您的问题，那么您想要做的就是根据角色名称生成列别名。您不能在MySQL语句中使用变量作为列别名，但是可以构造一个准备好的语句：
SET @sql = (SELECT CONCAT( 'SELECT username, ',
GROUP_CONCAT('(role.id = ', id, ') AS ', role SEPARATOR ', '),
' FROM user_role ',
'JOIN user ON user.id = user_role.user_id ',
'JOIN role ON role.id = user_role.role_id;')
FROM role);
SELECT @sql;
+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| @sql |
+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| SELECT username, (role.id = 1) AS Super, (role.id = 2) AS Admin, (role.id = 3) AS View, (role.id = 4) AS User, (role.id = 5) AS Email FROM user_role JOIN user ON user.id = user_role.user_id JOIN role ON role.id = user_role.role_id; |
+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
如您从输出中所见，它将生成一个包含SQL SELECT语句的字符串。现在，您需要从该字符串创建一个准备好的语句，并执行结果：
PREPARE stmt FROM @sql;EXECUTE stmt;
+----------+-------+-------+------+------+-------+
| username | Super | Admin | View | User | Email |
+----------+-------+-------+------+------+-------+
| Bob | 1 | 0 | 0 | 0 | 0 |
| Alice | 0 | 1 | 0 | 0 | 0 |
| Carol | 0 | 0 | 1 | 0 | 0 |
| Dave | 0 | 0 | 0 | 1 | 0 |
| Eve | 0 | 0 | 0 | 0 | 1 |
+----------+-------+-------+------+------+-------+
为了使调用交叉表查询更容易，您可以将整个过程包装在存储过程中。在以下示例中，如上所述，我无法GROUP_CONCAT在SET
@sql语句内工作。相反，我不得不将其分成自己的变量。我不确定为什么这行不通，但是最终结果是相同的，并且代码的神秘性可能有所降低：
DELIMITER //DROP PROCEDURE IF EXISTS test.crosstab//
CREATE PROCEDURE test.crosstab()
BEGIN
SET @cols = (SELECT GROUP_CONCAT(
'(role.id = ', id, ') AS ', role
SEPARATOR ', ') FROM role);
SET @sql = CONCAT(
'SELECT username, ',
@cols,
' FROM user_role ',
'JOIN user ON user.id = user_role.user_id ',
'JOIN role ON role.id = user_role.role_id;');
PREPARE stmt FROM @sql;
EXECUTE stmt;
END;
//
DELIMITER ;
CALL test.crosstab();
+----------+-------+-------+------+------+-------+
| username | Super | Admin | View | User | Email |
+----------+-------+-------+------+------+-------+
| Bob | 1 | 0 | 0 | 0 | 0 |
| Alice | 0 | 1 | 0 | 0 | 0 |
| Carol | 0 | 0 | 1 | 0 | 0 |
| Dave | 0 | 0 | 0 | 1 | 0 |
| Eve | 0 | 0 | 0 | 0 | 1 |
+----------+-------+-------+------+------+-------+
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