C# linq left join

左连接查询
seleft a.*,b.column1,b.column2 from a
left join b
on a.id = b.id

linq双表，多表连接查询

var list = from x in db.lefttable 
		   join y in db.righttable on x.id equals y.id into temp
		   from tt in temp.DefaultIfEmpty()
		   select new res {name=x.name,value=y.value};

var list2 = from x in db.lefttable 
			join y in db.righttable on x.id equals y.id into xyjoin 
			from xy in xyjoin.DefaultIfEmpty()
				join z in db.threetable on u.otherid equals xy.id into xyzjoin 
			from v in xyzjoin.DefualtIfEmpty() 
			select new {id = x.id,name = y.name,zvalue = z.value};