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.
Hi @gvwilson,

thank you for your comments. I was having troubles with understanding the original sentence.

After reading this about thirty times, I only have doubts regarding "input set" - what's that precisely? Is it any data in `person` column? Or any data in `person` column for which `quant='sal'` and `reading<=1.0`?

I'm guessing the latter. Is the naming "input set" correct? As a programmer I understand input as, for example, some sort of parameters bounded to a function on runtime.

All things considered, now I'm inclined towards this explanation:

> The answer is that the database manager isn't told how to aggregate that field (`person`). Therefore the manager can choose arbitrary value from the field `person` in records that satisfy above criteria.

What do you think, Greg?

Luca (cc @lucacerone), your input is also valuable!