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                            ## 1. key

~~~
DROP TABLE IF EXISTS `module_role`;
CREATE TABLE `module_role` (
  `rid` int(11) DEFAULT NULL,
  `mid` int(11) DEFAULT NULL,
  KEY `rid` (`rid`),
  KEY `mid` (`mid`),
  CONSTRAINT `mid` FOREIGN KEY (`mid`) REFERENCES `module` (`mid`),
  CONSTRAINT `rid` FOREIGN KEY (`rid`) REFERENCES `role` (`rid`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
~~~
>  KEY `rid` (`rid`),
>   KEY `mid` (`mid`),

key是用來索引約束的，也就是兩個作用：
1. 索引：
為表添加索引，索引的作用就是增加數據庫的查詢速度

2. 約束：
偏重于約束和規范數據庫的結構完整性
                    
                
        
                
            
        
                
    
                
    
                
        
                
    
                

                







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