Convert single row into multiple rows Bigquery SQL - sql

Question

Convert a row into multiple rows in bigQuery SQL.
The number of rows depend on a particular column value (in this case, the value of delta_unit/60):
Source table:
ID time delta_unit
101 2019-06-18 01:00:00 60
102 2019-06-18 01:01:00 60
103 2019-06-18 01:03:00 120
The ID 102 does recorded a time at 01:01:00 and the next record was at 01:03:00.
So, we are missing a record that should have been 01:02:00 and the delta_unit = 60
Expected table:
ID time delta_unit
101 2019-06-18 01:00:00 60
102 2019-06-18 01:01:00 60
104 2019-06-18 01:02:00 60
103 2019-06-18 01:03:00 60
A new row is created based on the delta_unit. The number of rows that need to be created will depend on the value delta_unit/60 (in this case, 120/60 = 2)

Answer 1

I have found a solution to your problem. I have done the following, first run
SELECT max(delta/60) as max_a FROM `<projectid>.<dataset>.<table>`
to compute the maximum number of steps. Then run the following loop
DECLARE a INT64 DEFAULT 1;
WHILE a <= 2 DO --2=max_a (change accordingly)
INSERT INTO `<projectid>.<dataset>.<table>` (id,time,delta)
SELECT id+1,TIMESTAMP_ADD(time, INTERVAL a MINUTE),delta-60*a
FROM
(SELECT id,time,delta
FROM `<projectid>.<dataset>.<table>`
)
WHERE delta > 60*a;
SET a = a + 1;
END WHILE;
Of course this is not efficient enough but it gets the Job done. The IDs and deltas do not finish at the right values yet, they should not be needed. The deltas would end up all at 60 (the column can be deleted) and the IDs can be recreated using the timestamp to get them ordered.
You might try using a conditional expression in here to avoid the loop and only going through the table once.
I have tried
INSERT INTO `<projectid>.<dataset>.<table>` (id,time,delta)
SELECT id+1, CASE
WHEN delta>80 THEN TIMESTAMP_ADD(time, INTERVAL 1 MINUTE)
WHEN delta>150 THEN TIMESTAMP_ADD(time, INTERVAL 2 MINUTE)
END
,60
FROM
(SELECT id,time,delta
FROM `<projectid>.<dataset>.<table>`
)
WHERE delta > 60;
but fails because only returns the first condition where the when is True. So, I am not sure if it is possible to do it all at once. If you have small tables I would stick to the first one which works fine.