如何根据变量选择模式？

Question

考虑：

SET @PREFIX='DEV_'; 

SET @REFRESHDB=CONCAT(@PREFIX,'Refresh'); 

CREATE TABLE @REFRESHDB.`Metadata` 
(
    `Key` VARCHAR(30) NOT NULL, 
    `Value` VARCHAR(30) NOT NULL, 
    PRIMARY KEY (`Key`) 
) ENGINE = InnoDB; 

INSERT INTO @REFRESDB.`Metadata` (`Key`, `Value`) VALUES ("Version", "0"); 

这似乎不是有效的：mysql回来了：

你在你的SQL语法错误;检查对应于你的MySQL服务器版本正确的语法使用近“@ REFRESHDB.`Metadata`

至于我可以告诉我正确地根据the documentation做的事情的手册。但MySQL说这是不允许的。这是MySQL的一些限制（不允许使用变量作为标识符）或其他？

Answer 1

文档状态：

“用户变量可以被分配一个值一组有限的数据类型：整型，十进制，浮点型，二进制或非二进制字符串，或NULL值“

您正在尝试将该变量用作对象。这不支持。

Answer 2

你将不得不使用准备语句/动态sql来做到这一点。

本文将详细说明这二者都在出色的细节：

http://rpbouman.blogspot.com/2005/11/mysql-5-prepared-statement-syntax-and.html

试试这个：

SET @PREFIX='DEV_'; 

SET @REFRESHDB=CONCAT(@PREFIX,'Refresh'); 

SET @st = CONCAT('CREATE TABLE ', @REFRESHDB,'.`Metadata` 
(
    `Key` VARCHAR(30) NOT NULL, 
    `Value` VARCHAR(30) NOT NULL, 
    PRIMARY KEY (`Key`) 
) ENGINE = InnoDB'); 

PREPARE tStmt FROM @s; 
EXECUTE tStmt; 


SET @s = CONCAT('INSERT INTO ', @PREFIX, '.`Metadata` (`Key`, `Value`) VALUES ("Version", "0")'); 

PREPARE stmt FROM @s; 
EXECUTE stmt; 

Answer 3

我建议你写一个存储过程：

DELIMITER $$ 
CREATE PROCEDURE (IN DBname varchar(255) 
       , IN AKey varchar(255) 
       , IN AValue varchar(255)) 
BEGIN 
    DECLARE query VARCHAR(1000); 
    -- First check the DBName against a list of allowed DBnames, 
    -- to prevent SQL-injection with dynamic tablenames. 

    DECLARE NameAllowed BOOLEAN; 
    SELECT 1 INTO NameAllowed WHERE DBName IN ('validDB1','validDB2'); 
    IF (NameAllowed = 1) THEN BEGIN 

    -- DBName is in the whitelist, it's safe to continue. 
    SET query = CONCAT('INSERT INTO ' 
         ,DBName 
         ,'.MetaData (`key`,`value`) values (?,?)); 
    -- note the use of parameter placeholders, to prevent SQL-injection. 
    PREPARE stmt FROM query; 
    EXECUTE stmt USING Akey, AValue; 
    DEALLOCATE PREPARE stmt; -- clears the query and its result from the cache. 
    END; END IF; 
END $$ 

DELIMITER ;