Currently it seems like there is no way to return a subset of items in a collection.  If I wanted to find items in a collection that matched some type of 'filter', I would have to retrieve all the items in the collection and filter myself to get this subset.  Here is API docs:

https://developers.webflow.com/#get-all-items-for-a-collection

For example, lets say I have a site that lists Albums by artists.  So I have a 2 collections:

1)  Artists

2)  Albums

One of the fields in the Album collection is 'artist_id'.  Lets say my favorite artist is Taylor Swift, which it is, there is no way for me to search the Album collection for:  artist_id = taylor_swift_id.

I would think this is a functionality that many developers would love to see.