I am really new to n8n as you will see. So here is my situation.
I query a rest api and get an array as answer. The next day i will do the same but want to compare the result to the day before to see if there are some new values.
To save the data you can use static-data if it is not large. Probably best you do not save the whole array only some kind of ID for each. That makes it then also easier to compare later to filter out the ones you did already process.
Here some information about static-data:
Here also some other ways to store data:
Apart from that should you find multiple examples if you search here in the forum for “filter”.