var results = from f in db.Farms 
                join fc in db.FarmCategories on f.FarmId equals fc.FarmId
                where f.FarmId.Equals(2)   
               select f;  

Left Join 在Linq中的写法如下，后面使用 DefaultIfEmpty()

var results = from f in db.Farms 
             join fc in db.FarmCategories.DefaultIfEmpty() on f.FarmId equals fc.FarmId
            where f.FarmId.Equals(2)
              select f;  

取实体类集合中某个字段值最大的那个实体类

ObservableCollection<RainFall> lists = e.Result;
 //从集合中找出最大值的实体
RainFall max = lists.OrderByDescending(s => s.YL).FirstOrDefault();