inner join 模板：

1. var query = from x in db.T1
2.             join y in db.T2
3.             on x.Id equals y.Id
4.             select x;

left join模板：

1. var query = from x in db.T1
2.             join y in db.T2
3.             on x.yId equals y.Id into temp
4.             from ret in temp.DefalutIfEmpty()
5.             select new{ x.Id, ret};

经常记不住,这次把它记下来!