查询格式如下：
var result = (from p in Package
join q in PackageLocationPrices on p.Id equals q.PackageId into pq
from r in pq.DefaultIfEmpty()
select new {p, r}).ToList();
标签：ToList,pq,多个,EF,查询,链表 From： https://www.cnblogs.com/microsoft-zh/p/17317694.html