GraphQL; select all records

Question

The service provider for an ERP software recently implemented GraphQL for those customers who wish to use their data beyond the tools they provide. I was asked to investigate its usefulness in creating digests of this data into more user-friendly data formats (excel, google sheets, ect ... )

While experimenting with the API using Altair I've run across a situation I don't understand, as I'm approaching the challenge as I would a RDMS:

{
  __type(name:"Jobs") {
  fields {
    name
    description
  }
  }
}

The above returns all the fields for the object 'Jobs'. For this examples, it returns a list of fields: JobNo, PartNo, PartRev, CustomerID, DueDate

I wanted to return a list of existing 'JobNo' in the Jobs object, so I tried the query:

{
    Jobs {
    JobNo DueDate
  }
}

Which resulted in:

{
  "errors": [
    "Expected a non-null value for argument \"JobNo\" on field \"Jobs\"."
  ]
}

If I use the following query:

{
  Jobs(JobNo:"12345")
  {
    JobNo DueDate
  }
}

I get results, but its not very useful as I would have to know something about the contents of the data.

Is there a way of querying for ALL JobNo? Or is the purpose of GraphQL to know at least some specifics of the data, like the JobNo, before constructing your query?

Thank you.