Mognodb displays all existing fields

  mongodb, question

Because mongo is a document type database, does a field have some records or not, then I want to know all the existing records in this tablefield nameHow do you get it?

For example:
A record has fields a,b,c b, c.
B records fields a, b.
C record has fields a,d,f d, f.

The desired results are all obtainedfield name:a,b,c,d,f
I think this watch has allfield nameHow?

One more question, mongo has no field comments. How do you manage it?
Thank you!

Not directly. find () can find out all the data

You can use the $exists keyword to find out all the data of a certain field: db.table.find (“_ id”: {$ exists: true}})