How to merge records based on consective fields in Teradata - teradata

Question

I have a source like below table:
+---------+--+--------+--+---------+--+--+------+
| ID | | SEQ_NO | | UNIT_ID | | | D_ID |
+---------+--+--------+--+---------+--+--+------+
| 7979092 | | 1 | | 99 | | | 759 |
| 7979092 | | 2 | | -1 | | | 869 |
| 7979092 | | 3 | | -1 | | | 927 |
| 7979092 | | 4 | | -1 | | | 812 |
| 7979092 | | 5 | | 99 | | | 900 |
| 7979092 | | 6 | | 99 | | | 891 |
| 7979092 | | 7 | | -1 | | | 785 |
| 7979092 | | 8 | | -1 | | | 762 |
| 7979092 | | 9 | | -1 | | | 923 |
+---------+--+--------+--+---------+--+--+------+
I have to merge the rows when consecutive unit_id has same value. We should take max(D_id) when we consolidate the rows. Expected output is:
+---------+---------+------+
| ID | UNIT_ID | D_ID |
+---------+---------+------+
| 7979092 | 99 | 759 |
| 7979092 | -1 | 927 |
| 7979092 | 99 | 900 |
| 7979092 | -1 | 923 |
+---------+---------+------+
I have tried to find the solution using Teradata ordered analytical function, but did not find the solution. I use Teradata 16.
Thank You.

Answer 1

This logic is a bit quirky, it's based on two sequences created by different sort orders:
SELECT
ID
,UNIT_ID
,Max(D_ID)
FROM
(
SELECT
ID
,SEQ_NO
,UNIT_ID
,D_ID
-- assign the same value to consecutive UNIT_IDs
,SEQ_NO -
Row_Number()
Over(PARTITION BY ID, UNIT_ID
ORDER BY SEQ_NO) AS grp
FROM tab
) AS dt
GROUP BY 1,2,grp

Answer 2

You can use RESET WHEN to dynamically create groups within the window. Here's one way to do it:
select ID, UNIT_ID,
max(D_ID) over(
partition by ID order by SEQ_NO
reset when UNIT_ID <> UNIT_ID_prev -- Create new group for new value
) as D_ID
from (
select ID, SEQ_NO, UNIT_ID, D_ID,
lag(UNIT_ID) over(partition by ID order by SEQ_NO) as UNIT_ID_prev -- Previous value
from MY_TABLE
) src
qualify row_number() over(
partition by ID order by SEQ_NO
reset when UNIT_ID <> UNIT_ID_prev -- Match original max() window
) = 1 -- One row per group (similar to DISTINCT)