Swc novice sql

These are the modifications @pbanaszkiewicz and I agreed on as part of the Software Carpentry teaching session.

Piotr added a nice figure to lesson 1 showing graphically what fields, columns and records are.
We also modified a bit the text and clarifiied how the count() aggregator works in lesson 6.
How about this:

The answer is that when the database manager has to aggregate a field,
but isn't told how to do so,
it's allowed to choose any value it wants to from its input.
It could use the first one it sees,
the last one,
or something else entirely,
and it could choose something different the next time the query is run.