将查询拆分为多个部分,并查看哪个集合没有元素。
var mapList = DtMapGuestDepartment.AsEnumerable().ToList();
var deptList = DtDepartment.AsEnumerable().ToList();
var queryResult1 = (
from map in mapList
where map.Field<Nullable<long>>("GUEST_ID") ==
DRowGuestPI.Field<Nullable<long>>("PK_GUEST_ID")
select map
).ToList();
var queryResult2 = (
from map in queryResult1
join dept in deptList
on map.Field<Nullable<long>>("DEPARTMENT_ID")
equals dept.Field<Nullable<long>>("DEPARTMENT_ID")
select dept.Field<string>("DEPARTMENT_ID")
).ToList();