可以用 into 语法，配合 DefaultIfEmpty() 方法加载右侧表中的空值。
例子：
var query =
from r in db.Roles
join sr in
(
from ss in db.StaffInRoles
where ss.StaffId == staffId
select ss
)
on r.Id equals sr.RoleId
into x
from y in x.DefaultIfEmpty()
select new RoleInfo
{
RoleId = r.Id,
RoleName = r.Name,
IsSet = y != null
};