Dataset with no members

I am trying to plan a framework for relational algebra using the Dataset capability of Mathematica 10.

In the work of Date and Darwen, there are two concepts known as TABLE_DEE and TABLE_DUM. TABLE_DEE is a relation with a (column) header but no rows, whereas TABLE_DUM is simply:

Dataset[{}]

How might I create a TABLE_DEE with Dataset--a table with headers, but no data? I feel this would allow a framework for performing orthodox relational database operations.

Answer

You have to use the undocumented (and subject-to-change) syntax for specifying a type or schema:

Needs["TypeSystem`"];
Dataset[
  {}, 
  Vector @ Struct[

    "field1" -> Atom[Integer], 
    "field2" -> Atom[String]
  ]
]

Replace the contents of the inner Struct as you see fit (you can always use TypeSystem`DeduceType on a chunk of data obeying your desired schema to see more examples of these type expressions).

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