linq多条件left join
equals两边的参数字段名的大小写必须完全匹配
var propID = 1;
var list = from t in tt
join detail in OrderDetailList
on new { ID = t.ID, PropId = propID } equals new { ID = detail.OrderID, PropId = detail.PropID } into od
from dieDetail in od.DefaultIfEmpty()
orderby dieDetail.PropValue
select t;
