[feature] runningDifference function

[BohuTANG]

Summary

Calculates the difference between successive row values ​​in the data block.

Like a table repo_stars:

date	stars
2021-09-10	1035
2021-09-09	1021
2021-09-08	1000

SELECT date, runningDifference(start) as delta FROM repo_stars

date	delta
2021-09-10	14
2021-09-09	21
2021-09-08	0

[ygf11]

/assignme

[ygf11]

It seems runningDifference function should consider null value condition, and the behavior in clickhouse looks like:

1. if row[i] is null, then the result is null.
2. if row[i] is not null, and row[i-1] is null, then the result is 0.

is this the expected logic?