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Explain NULLs when teaching count #324

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Explain NULLs when teaching count #324

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Explain NULLs when teaching count
chrisroat Sep 14, 2020
7fff12b
Clarify how count works
chrisroat Sep 15, 2020
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13 changes: 9 additions & 4 deletions _episodes/06-agg.md
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Expand Up @@ -95,11 +95,16 @@ SELECT sum(reading) FROM Survey WHERE quant = 'sal';
|64.83 |

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(*)`,
but could have used `count(*)`,
since the function doesn't care about the values themselves,
just how many values there are.
just how many rows there are.
Even a column other than `reading` could be used,
but note that any `NULL` value will not be counted
(to see, try `count`ing the `person` column, which contains a
row with a `NULL`).
This perhaps non-obvious behavior
of aggregation functions is covered later
in this episode.

SQL lets us do several aggregations at once.
We can,
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