动态MySQL查询/视图交叉

Question

目前，我有以下SQL硬编码的视图：

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，它是否可以动态生成中的作用从表中记录的角色列。

Answer 1

你可以做你想做的事，但我不确定为什么你会想。一旦你有你的动态列别名，你如何计划引用它们？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？我可能会错过你的问题背后的原因。

反正我假设你有一个这样的结构：

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); 

从这一点，你可以获取有关用户和他们的角色（一个或多个）信息：

SELECT username, role.id AS role_id, role.role AS role FROM user_role 
JOIN 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 | 
+----------+---------+-------+ 

您还可以创建一个列别名为特定角色：

SELECT username, (role.id = 1) AS Super FROM user_role 
JOIN 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，如果我正确理解您的问题，你想要做的是从角色名称中生成列别名。你不能用一个变量作为一个MySQL声明列别名，但你可以构造一个事先准备好的声明：

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; | 
+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ 

，你从输出，其生成包含SQL SELECT语句的字符串见。现在，您需要创建从该字符串一份声明，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 | 
+----------+-------+-------+------+------+-------+ 

编辑

为了调用交叉表查询更方便，你可以换了整个事情了在存储过程。在以下示例中，我无法使GROUP_CONCAT在SET @sql声明中工作，如上所述。相反，我必须把它分解成它自己的变量。我不知道为什么没有工作，但最终的结果是一样的，而且代码可能少一点神秘：

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 | 
+----------+-------+-------+------+------+-------+