[bni]

What about when you want to find all notes that was made a specific day last month (say for a report)?

Traverse all notebook documents and look at each notes date? Good luck with that.

[artsrc]

Many nosql databases support queries. If you are using one of these then you are in a better place for features like that than you are with heavily shaded SQL.

[tommyd]

If you were to use MongoDB, wouldn't this just be a case of adding an index to the field of the nested child document (i.e. the note within a notebook) you were interested in and then querying on it? e.g. db.notebooks.ensureIndex({"notes.date": 1});