Is it possible with ODK to send specific fields from the app to the server? not all questions.
some time I create fields, but I do not want to send it to the server and make the database cluttered.
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No, all the fields defined in the form will be sent to the server. An insert with null values for unused fields will be performed.
Anyway the database tables are created when you push your blank form to aggregate, with columns for all the fields of your form.