How to concatenate N rows of a column based on a specific column value using a condition in Google BigQuery?

Question

TimeStamp	USER ID	string_col
1	100001	Here
2	100001	there
3	100001	Apple
4	200002	this is
5	200002	that is
6	200002	Apple
7	200002	some
8	200002	summer
9	200002	winter
10	200002	Apple

That is my raw table & I want to use the word "Apple" as a trigger word to concatenate ONLY N (For this example 2) rows above that for every userID.

Below table is the output I'm looking for:

TimeStamp	USER ID	string_col	Result
1	100001	Here	null
2	100001	there	null
3	100001	Apple	Here There
4	200002	this is	null
5	200002	that is	null
6	200002	Apple	this is that is
7	200002	some	null
8	200002	summer	null
9	200002	winter	null
10	200002	Apple	summer winter

Answer 1

For the passion of problem solving and coding, I would like to share this simple solution to see if it helps to answer your question:

SELECT
  t1."TimeStamp",
  t1."USER ID",
  t1."string_col",
  (
    SELECT
      STRING_AGG(t2."string_col", ' ')
    FROM
      raw_table AS t2
    WHERE
      t2."USER ID" = t1."USER ID"
      AND t2."TimeStamp" < t1."TimeStamp"
      AND t1."string_col" = 'Apple'
      AND t2."string_col" != 'Apple'
      AND t1."TimeStamp" - t2."TimeStamp" <= 2
  ) AS Result
FROM
  raw_table AS t1

The output looks like this:

Answer 2

Use below

SELECT * EXCEPT(grp),
  CASE 
    WHEN string_col = 'Apple'
    THEN STRING_AGG(string_col, ' ') OVER win
  END AS Result
FROM (
  SELECT *, 
    COUNTIF(string_col = 'Apple') OVER win AS grp
  FROM your_table
  WINDOW win AS (PARTITION BY user_id ORDER BY ts ROWS BETWEEN UNBOUNDED PRECEDING AND 1 PRECEDING)
)
WINDOW win AS (PARTITION BY user_id, grp ORDER BY ts ROWS BETWEEN 2 PRECEDING AND 1 PRECEDING)     

with output