Compare data from different records

I need to calculate the difference between data from different records.
I have a table with articles, with a unique code that I use as Index called CN.
In another table I have dates with stocks of each of the articles.
In this table, the stocks of each item are not correlative because record number 1 has the stock of item 1 on day 1 and record 2 contains the stock of item 2 on day 1, so on until record n contains the stock of article 1 on day 2, n + 1 the stock of article 2 on day 2, etc ...

I need to calculate the difference between the stock of an item between two consecutive days.

Any ideas?

Thank you very much and greetings