JDBC： SQL语句恶心的错误之 字符串拼接少空格 导致SQLSYNTAXEXCEPTION

private Student searchStudent(int searchType) {        String sql="SELECT flowid,type,idcard,examCard,studentName"                + ",location,grade"                + "FROM examstudent "                + "WHERE ";                Scanner scanner=new Scanner(System.in);//        1.根据输入的searchType确定查询的方式//        2.根据searchType确定sql语句        if(searchType==1){            System.out.print("请输入身份证号：");            String idCard=scanner.next();            sql=sql+"idcard=‘"+idCard+"‘";        }else{            System.out.print("请输入准考证号：");            String examCard=scanner.next();            sql=sql+"examCard=‘"+examCard+"‘";        }