1.1 编写sql实现每个用户截止到每月为止的最大单月访问次数和累 计到该月的总访问次数 userid,month,visitsA,2015-01,5A,2015-01,15B,2015-01,5A,2015-01,8B,2015-01,25A,2015-01,5A,2015-02,4A,2015-02,6B,2015-02,10B,2015-02,5A,2015-03,1