1、单张表：var q =      from p in db.Products      group p by p.CategoryID into g      select g;  2、多表多字段参与分组：from a in TableA      join b in TableB on a.Id equals b.aId      where ((b.Type == 1 || b.Type == 2 || b.Type == 5) && b.State == 1)      group new { a.Id, b.Name,b,CreateDate } by new { a.Id, b.Name } into g      select (new MyClass { Id = g.Key.Id, Name = g.Key.Name ?? "" });    class MyClass {      public int Id { get; set; }      publid string Name { get; set; }  }