LINQ 左联查询 Left Join

            //左连接,left join
            var listSource = (from a in q
                              join b in q_AccountItems on a.AccountId equals b.ItemID into tmp
                              from b in tmp.DefaultIfEmpty()
                              select new
                              {
                                  isid = a.isid,
                                  YYYYMM = a.DocDate.Value.Year.ToString() + a.DocDate.Value.Month.ToString().PadLeft(2, '0'),
                                  ItemId = a.AccountId,
                                  ItemName = b != null ? b.ItemName : "",
                                  Amount = a.ARAP == "收" ? a.Amount : (a.Amount ?? 0) * -1,//收：正数，支：负数
                              }).ToList();