Time difference between rows based on condition

Not really an expert with SQL and im having problems figuring out how to do this one.

Got a table like this one:

I need to calculate the time difference each time there is a response from the B user after a message from A.

Expected result would be like this

Trying to use Lead function to obtain the next desired timestamp but im not getting the expected result Any tips or advice?

Thanks

Even if it is already answered - it's a nice use case for Vertica's MATCH() clause. Looking for a pattern consisting of sender = 'A' followed by sender = 'B'. You get a pattern id, and then you can group by other stuff plus the pattern id to get max timestamp and min timestamp.

Also note that I renamed both "user" and "timestamp", as they are reserved words...

-- your input, don't use in final query
indata(ID,Message,ts,Usr) AS (
          SELECT 1,'Hello'       ,TIMESTAMP '2022-08-01 10:00:00','A'
UNION ALL SELECT 1,'How are you?',TIMESTAMP '2022-08-01 10:00:05','A'
UNION ALL SELECT 1,'Hello there' ,TIMESTAMP '2022-08-01 10:00:10','B'
UNION ALL SELECT 1,'I am okay'   ,TIMESTAMP '2022-08-01 10:00:12','B'
UNION ALL SELECT 1,'Good to know',TIMESTAMP '2022-08-01 10:00:15','A'
UNION ALL SELECT 1,'Bye'         ,TIMESTAMP '2022-08-01 10:00:25','B'
UNION ALL SELECT 2,'Hello'       ,TIMESTAMP '2022-08-01 10:02:50','A'
UNION ALL SELECT 2,'Hi'          ,TIMESTAMP '2022-08-01 10:03:50','B'
)
-- end of input, real query starts here , replace following comma with "WITH" 
,
w_match_clause AS (
  SELECT                                                                                                                                             
    *
  , event_name()
  , pattern_id()
  , match_id()
  FROM indata
  MATCH (
    PARTITION BY id ORDER BY ts
    DEFINE
      sentbya AS usr='A'
    , sentbyb AS usr='B'
    PATTERN
    p AS (sentbya sentbyb)
  )
   -- ctl SELECT * FROM w_match_clause;
   -- ctl  ID |   Message    |         ts          | Usr | event_name | pattern_id | match_id 
   -- ctl ----+--------------+---------------------+-----+------------+------------+----------
   -- ctl   1 | How are you? | 2022-08-01 10:00:05 | A   | sentbya    |          1 |        1
   -- ctl   1 | Hello there  | 2022-08-01 10:00:10 | B   | sentbyb    |          1 |        2
   -- ctl   1 | Good to know | 2022-08-01 10:00:15 | A   | sentbya    |          2 |        1
   -- ctl   1 | Bye          | 2022-08-01 10:00:25 | B   | sentbyb    |          2 |        2
   -- ctl   2 | Hello        | 2022-08-01 10:02:50 | A   | sentbya    |          1 |        1
   -- ctl   2 | Hi           | 2022-08-01 10:03:50 | B   | sentbyb    |          1 |        2
)
SELECT
  id
, MAX(ts) - MIN(ts) AS difference
FROM w_match_clause
GROUP BY
  id
, pattern_id
ORDER BY
  id;
-- out  id | difference 
-- out ----+------------
-- out   1 | 00:00:05
-- out   1 | 00:00:10
-- out   2 | 00:01