tk.mybatis的selectOne需注意默认值问题，否则会隐式增多属性

        Clerk clerk = new Clerk();
        clerk.setName(name);
        clerk.setPassword(password);
        return clerkMapper.selectOne(clerk);    

此时对应SQL语句：SELECT name,password,telephone,city,skills,state FROM clerk WHERE name = ? AND password = ?

参数：hh(String), 123(String)

如果Clerk实体中有一个属性加了默认值（比如 private String skills = "";）那么即使没有clerk.setSkills("")，SQL语句也会成为

SELECT name,password,telephone,city,skills,state FROM clerk WHERE name = ? AND password = ? AND skills = ?

参数：hh(String), 123(String)，(String)