What is the BeatmapSetID of a map that doesnt contain BeatmapSetID

[leryss]

I've downloaded an archive of osu maps from data.ppy.sh.
When I started parsing all these maps, I've found that there are a lot without a BeatmapSetID.
Old maps I'm guessing. Do they have a set id? If they dont have a set id can I find them on the official website with a format similar to "https://osu.ppy.sh/beatmapsets/{BeatmapSetID}#osu/{BeatmapID}" ?

[peppy]

You will need to query this via the API yourself.