您需要做的是这样的：

 var result = from at in db.AssetTagging
              join r in db.Returns on at.AssetID equals r.AssetID into a
              from returns into a.DefaultIfEmpty()
              join i in db.Issues on at.AssetID equals I.AssetID into b
              from issues into b.DefaultIfEmpty()
              where issues.AssetID != null || returns.RequestStatus == "Approved"
              select new
                    {
                      AssetID = at.AssetID,
                      AssetDescription = at.AssetDescription,
                      Status = returns != null ? returns.RequestStatus : null
                    }.ToList();