比如
job(job_id,job_user_id,job_operator_id)
两个实体：
jobuser(job_user_id.job_user_name)
job_operator(job_operator_id,job_operator_name)
我要查的是
job_id ,job_user_id,job_user_name,job_operator_id,job_operator_name)
单个字段的leftjoin我知道，但是多个的我不知道应该怎么写
------解决思路----------------------
from a in db.A
join b in db.B on new {C=a.A,D=a.B} equals new {C=b.A,D=b.B} into j1
from b in j1.DefaultIfEmpyt()
join c in db.C on a.C equals c.C into j2
from c in j2.DefaultIfEmpty
select ……