Help to find in sequence issue in groups records

1. Add a continued sequence with ROW_NUMBER, named PP_seq_correct;

2. Calculate differences of PP_seq and PP_seq_correct, get the difference of first element in each group_cost set as stand value.

3. Filter the rows that the differences not equal the stand value, they are all incorrect.

4. Get the first row from the incorrect rows grouping by group_cost.

WITH T (group_cost, PP_Seq, Seq_no, diff) AS
(
  SELECT c.group_cost, c.PP_seq, a.PP_seq_correct, c.PP_seq-a.PP_seq_correct AS diff 
  FROM #Cost c
  INNER JOIN
  (
    SELECT group_cost, PP_Seq, ROW_NUMBER() OVER(PARTITION BY group_cost ORDER BY PP_seq) AS PP_seq_correct FROM #Cost
  ) a
  ON c.group_cost=a.group_cost AND c.PP_seq=a.PP_seq
)
SELECT Co.*, 'Comments...' Comments FROM #cost Co INNER JOIN
(
  SELECT T.group_cost, Min(PP_Seq) PP_Seq FROM T LEFT JOIN
  (
    SELECT T.group_cost, T.diff FROM T WHERE T.seq_no=1
  ) D
  ON T.group_cost=D.group_cost AND T.diff=D.Diff
  WHERE D.diff IS NULL
  GROUP BY T.group_cost
)R
ON Co.group_cost=R.group_cost AND Co.PP_Seq=R.PP_Seq