Retrieving all entries 10 days prior to days in table, which matches where clause - kdb

Question

I have a random table of data - with dates and numbers:
Date Open Volume abschange
2016.12.08D00:00:00.000000000 11035.76 1.74835e+008 1.30177
2016.12.09D00:00:00.000000000 11170.18 1.0383e+008 0.2994598
2016.12.12D00:00:00.000000000 11198.42 8.98117e+007 0.07331357
...
2016.12.30D00:00:00.000000000 11443.31 4.18109e+007 0.3298871
2017.01.02D00:00:00.000000000 11426.38 4.74561e+007 1.504853
So from this table, i would like to be able to create a list, which holds all the entries from 10 days prior to the days, which has abschange>1.
I thought it would be easiest to start with a focus on those dates:
Date abschange
---------------------------------------
2016.12.08D00:00:00.000000000 1.30177
2017.01.02D00:00:00.000000000 1.504853
2017.01.25D00:00:00.000000000 1.099709
2017.01.31D00:00:00.000000000 1.344625
2017.02.06D00:00:00.000000000 1.016427
2017.02.21D00:00:00.000000000 1.265196
Then create a flat list:
mynewdates: raze tablewithDateAndabschange each
which gives me:
2016.12.08D........ 2017.01.02D......
Then i get stuck, when i want to add 10 prior dates for each entry in this list.
Could i actually get my wanted result in one line of code, based on the first table or should i follow the path i am on ?
For both - if possible, what would the possible solution to this be ?

Answer 1

If my understanding is correct your requirement is:
for each date for which abschange>1, get last 10 dates before this date from table.
Below query will create that map. It is based on following assumptions:
Date column is unique and ordered(ascending) as it appears from your example.
If above condition is not true then it will require a minor change in below query to work with duplicates and unordered list.
Table is not keyed.
q) (tbl[`Date]a)!b#'where#'not null b:tbl[`Date] -1+(a:where 1<tbl`abschange)-\:til 10
UPDATE: Based on discussion in comment section.
Just add the second step to check if dates list for first result is empty. In that case generate last 10 dates from that date.
Finally it generates the table where each row contains prior dates(max 10) for each date with abschange>1.
q)d:b#'where#'not null b:tbl[`Date] -1+(a:where 1<tbl`abschange)-\:til 10
q)d[0]:$[0=count d 0;(t[`Date]a 0)-1+til 10;d 0]
q)([]dates:d)