Difference between rows in KDB/Q

I'm new to KDB/Q and have a question around getting the difference between two (not necessarily adjacent) rows. I have only one table, which looks like the below: ``` q)tickers:`ibm`bac`dis`gs`ibm`gs`dis`bac q)pxs:100 50 30 250 110 240 45 48 q)dates:2013.05.01 2013.01.05 2013.02.03 2013.02.11 2013.06.17 2013.06.21 2013.04.24 2013.01.06 q)trades:([tickers;dates];pxs) q)trades tickers dates | pxs ------------------| --- ibm 2013.05.01| 100 bac 2013.01.05| 50 dis 2013.02.03| 30 gs 2013.02.11| 250 ibm 2013.06.17| 110 gs 2013.06.21| 240 dis 2013.04.24| 45 bac 2013.01.06| 48 ``` I would like to be able to have a either another column in the table that stores the difference between the current and the previous price, or another structure similar in structure. The key question that the resulting needs to answer is "by how much did the stock change compared to the previous time a price was recorded?" So far I've tried something along the lines of: ``` select tickers, dates, pxs - pxs(dates bin (exec dates from trades where tickers = trades.tickers)) from trades ``` which doesn't really work (at all). Definitely due to trying to do SQL-like queries and having a row-oriented mindset. Please find below an exemple of the sought after answer: ``` q)trades: do magic with trades q)trades tickers dates | pxs | delta ------------------| --- | ----- ibm 2013.05.01| 100 | 0 bac 2013.01.05| 50 | 0 dis 2013.02.03| 30 | 0 gs 2013.02.11| 250 | 0 ibm 2013.06.17| 110 | 10 gs 2013.06.21| 240 | -10 dis 2013.04.24| 45 | 15 bac 2013.01.06| 48 | -2 ``` Thanks for your help, Dan