//按年分组
var year = db.News.GroupBy(g =>new { Year= g.RiQi.Year }).Select(s=>new { s.Key.Year});
//按年月
var yearMonth = db.News.GroupBy(g =>new { Year= g.RiQi.Year ,Month=g.RiQi.Month}).Select(s=>new { Keys = t.Key.Year.ToString() + "-" + t.Key.Month.ToString().Length >= 2 ? t.Key.Month.ToString() : "0" + t.Key.Month.ToString())});