ID database

I have begun to make an excel database to make easier identification of mammillaria sp.
Central Spine number
Central Spine colour
Central Spine length
Central Spine shape
Central Spine hooked or not
Radial Spine properties...
Seed properties ...
Flower properties ..
Fruit properties...
Body properties...
Axil properties...
Tubercule  properties...
But it is hard to prepare that database.

There are also online database websites.
Is it possible to make an online database with a team work?

I think that the challenge with your idea probably wouldn't be the actual technology, but obtaining all the data you would need. There's a mass of published but not immediately available data, and to assimilate, encode in a meaningful way, sounds quite daunting to me. The concept of being able to enter key characters of a plant that I had into a database search engine and come out with a single or maybe a few suggested species name for it would be very useful to many people, I suspect.
But is it achievable?

Jonas Lüthy has actually performed such an analysis. He has investigated 50+ properties of a whole range of mammillaria-species. See his 1995 thesis 'Taxonomische Untersuchung der Gattung Mammillaria'.
In my opinion, the variation in properties is so large that it will be very difficult to identify an unknown species by using your method.

Wolter ten Hoeve.

I'd forgotten that Luthy's thesis does go into quite a lot of detail. Its a long time since I looked at it, and I had remembered it as being not much more than explaining why he came to a certain classification of species in Mammillaria. It is this, of course, but also a whole lot more.

There's still a lot of validity in Luthy's classification, even though Hunt has revised his own twice now, I think, once for the first edition of the New Cactus Lexicon, and secondly more recently for an update which is now in the CITES checklist Version 3. Which just illustrates a big problem, that there is no consensus across the world really of a single taxonomic classification, mostly because of inherent variability in these plants and the interpretations of those variations by so-called "Authorities".

Chris I am now reading Luthy's, and Hunt classification. And Dave Ferguson approach. And 2006 – Classification : The Genus Mammillaria as per the New Cactus Lexicon. (at mammillarias.net).
I think it will be useful to know some of that clasifications. Thank you.

Is 2006 David Hunts "New Cactus Lexicon" valid ? I like that. Or whats the weaknes of that classification?

What is valid? I think that until such time as there is a major dna level study of the genus, there will be differing views as to how to divide it into species, subspecies etc. Those studies done so far all seem to agree that Mammillaria as we generally understand it is not a single genus, but that it isn't a simple matter just to split off those former genera like Cochemeia.

Until then, I think we amateur enthusiasts just have to accept that the varying classifications, and splitting or lumping, reflect different botanists view of the genus, done perhaps with different views of what defines a species or subspecies, or even genus!

Where a plant comes from helps to clarify its place in these classifications, which is what sometimes makes it really hard to identify a photo of a plant which doesn't conform to the "popular" view of a specific species. So, I urge everyone who wants to understand Mammillarias and grow them well, to understand where the specific plant comes from, ideally it should have a field collection reference, and also not be too distant in generations from the original collection (in case of accidental hybridisation).