Explain NULLs when teaching count

[chrisroat]

The removes a small oversight in the current explanation, that columns with NULL values are not counted. It also teases the fuller explanation of NULL and aggregation that comes later.

Closes #305

[henrykironde]

@r4space @remram44 any input on this?

[remram44]

"how many values there are" is still a bit ambiguous, it could be read as "how many distinct values" (count(distinct reading)).

[chrisroat]

Good point -- changed it to rows. Also better explained (I think) the NULL in the person column.

[chrisroat]

Any additional suggestions?

[chrisroat]

Any additional suggestions?

[chrisroat]

If this change is not desired, I can drop this pull request.

[PaulHancock]

I would suggest noting that all these aggregation functions ignore null values in their computations (otherwise avg, sum, would result in null).
For the most part we get exactly what we intend when running these functions, with the exception of count.

The count function will count the number of non-null entries when run on a column, but will count the number of rows when run with count(*) (even if the rows contain nulls).

So count(colname) should be used when you want to know the number of non-null entries, where as count(*) should be used to count the number of rows total.

The current text:

We used count(reading) here, but we could just as easily have counted quant or any other field in the table, or even used count(*), since the function doesn’t care about the values themselves, just how many values there are.

Is therefore not correct, and should be updated.

I think the changes in 4e1bcb5, and 7fff12b should be accepted.

[zkamvar]

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