String Concat over partition in SQL?

Question

I want to concatenate the surrounding rows(in the following examples only the surrounding 2 rows) after ranking to a new column(group by seems to not work), and here is the data I have:

Schema (MySQL v8.0)

CREATE TABLE log_table (
  `user_id` VARCHAR(5),
  `date_time` DATETIME,
  `event_name` VARCHAR(10),
  `trivial` int
);

INSERT INTO log_table
  (`user_id`, `date_time`, `event_name`, `trivial`)
VALUES
  ('001', '2020-12-10 10:00:02', 'c', 3),
  ('001', '2020-12-10 10:00:01', 'b', 9),
  ('001', '2020-12-10 10:00:40', 'e', 2),
  ('001', '2020-12-10 10:00:20', 'd', 6),
  ('001', '2020-12-10 10:00:00', 'a', 1),
  ('002', '2020-12-10 10:00:10', 'C', 9),
  ('002', '2020-12-10 10:00:50', 'D', 0),
  ('002', '2020-12-10 10:00:02', 'A', 2),
  ('002', '2020-12-10 10:00:09', 'B', 4);

---

To illustrate what I want to do. I can do summing over numerical values using the sum clause as follows:
Query #1

SELECT *,
       SUM(trivial)
         over(
           PARTITION BY user_id
           ORDER BY user_id, date_time ROWS BETWEEN 2 preceding AND 2 following)
       AS
       trivial_new
FROM   log_table; 

user_id	date_time	event_name	trivial	trivial_new
001	2020-12-10 10:00:00	a	1	13
001	2020-12-10 10:00:01	b	9	19
001	2020-12-10 10:00:02	c	3	21
001	2020-12-10 10:00:20	d	6	20
001	2020-12-10 10:00:40	e	2	11
002	2020-12-10 10:00:02	A	2	15
002	2020-12-10 10:00:09	B	4	15
002	2020-12-10 10:00:10	C	9	15
002	2020-12-10 10:00:50	D	0	13

---

View on DB Fiddle

For the string field event_name, I tried this snippet:
Query #2

SELECT *,
       Concat(event_name)
         over(
           PARTITION BY user_id
           ORDER BY user_id, date_time ROWS BETWEEN 2 preceding AND 2 following)
       AS
       event_name_new
FROM   log_table

And here is my expected results:

user_id	date_time	event_name	trivial	event_name_new
001	2020-12-10 10:00:00	a	1	abc
001	2020-12-10 10:00:01	b	9	abcd
001	2020-12-10 10:00:02	c	3	abcde
001	2020-12-10 10:00:20	d	6	bcde
001	2020-12-10 10:00:40	e	2	cde
002	2020-12-10 10:00:02	A	2	ABC
002	2020-12-10 10:00:09	B	4	ABCD
002	2020-12-10 10:00:10	C	9	ABCD
002	2020-12-10 10:00:50	D	0	BCD

But the Query #2 cannot get me here, and I have googled but all I can find is about group by(refer to this and this and this).

I know I can work around the problem by using LAG and LEAD(for the following rows) but I need to concatenate the new columns and when I need to concatenate many rows I need to do lots of manual work like concatenate them by separators like , and etc.

Can I do that in one step without using LAG and LEAD?

Answer 1

You can solve this problem by using CTEs: first computing row numbers for each user_id, ordered by date_time; then JOINing the table to itself based on the row number being within the minimum/maximum row number range for that row (which is from row number - before to row number + after). Then you can just GROUP_CONCAT the event_name field from the JOINed table:

SET @before := 2;
SET @after := 2;

WITH rns AS (
  SELECT *, 
         CAST(ROW_NUMBER() OVER (PARTITION BY user_id ORDER BY date_time) AS SIGNED) AS rn
  FROM log_table
)
SELECT r1.user_id, r1.date_time, r1.event_name,
       GROUP_CONCAT(r2.event_name SEPARATOR '') AS event_name_new
FROM rns r1
JOIN rns r2 ON r2.user_id = r1.user_id
           AND r2.rn BETWEEN r1.rn - @before AND r1.rn + @after
GROUP BY r1.user_id, r1.date_time, r1.event_name
ORDER BY r1.user_id, r1.rn

Output (for 2 before and 2 after):

user_id     date_time               event_name  event_name_new
001         2020-12-10 10:00:00     a           abc
001         2020-12-10 10:00:01     b           abcd
001         2020-12-10 10:00:02     c           abcde
001         2020-12-10 10:00:20     d           bcde
001         2020-12-10 10:00:40     e           cde
002         2020-12-10 10:00:02     A           ABC
002         2020-12-10 10:00:09     B           ABCD
002         2020-12-10 10:00:10     C           ABCD
002         2020-12-10 10:00:50     D           BCD

Demo on db-fiddle

Answer 2

A correlated subquery might be the simplest solution:

with l as (
      select l.*,
             cast(row_number() over (partition by user_id order by date_time) as signed) as seqnum
      from log_table l
     )
select l.*,
       (select group_concat(l2.event_name order by l2.date_time separator '')
        from l l2
        where l2.user_id = l.user_id and
              l2.seqnum between l.seqnum - 2 and l.seqnum + 2
       ) as new_event_name
from l;

If the event names are one character, you can eliminate the correlated subquery and use string operations:

with l as (
      select l.*, full_concat,
             cast(row_number() over (partition by user_id order by date_time) as signed) as seqnum
      from log_table l join
           (select user_id,  group_concat(event_name order by date_time separator '') as full_concat
            from log_table l
            group by user_id
           ) ll
           using (user_id)
     )
select l.*, substr(full_concat, greatest(seqnum - 2, 1), least(5, seqnum + 2))
from l;

Here is a db<>fiddle.