Mapper mapping file # and $ problems

 

        About the # and $ difference between 
        
        the SELECT * from the User the WHERE username = " ? " And password =? "
         
        # Number: 
            1 . Use the # pass parameters have pre-compiled an effect of preventing sql injection attacks
             2 . Add one pair for the parameter " " No 
            agreement: do not use the # $ 
        $ symbol: 
            only the name field is a parameter, just use the $ symbol 
            example: 
                the SELECT