json schema and sql table definition for data entity
All right, now it's clear that your actual requirement isn't about JSON. Your requirement is getting the field names, data types etc.
If you want to see an example of valid data, simply use the export in data management (to CSV, in your case).
To get additional details, check out whether the entity is public. If so, you can utilize OData metadata. As I already mentioned, you can even use it to generate code, so you don't have to worry about implementation details such as the datatime format, you'll get data type validations etc.
Even more details can be extracted with the metadata API in F&O. You might also be interested in AOTBrowser (an add-on to F&O), which utilizes this API.
A possibility of looking at the view definition or the staging table definition was already mentioned.