From 4e1bcb583b34ea498699476ee8f62322ffe9a731 Mon Sep 17 00:00:00 2001
From: Chris Roat <1053153+chrisroat@users.noreply.github.com>
Date: Mon, 14 Sep 2020 16:26:52 -0700
Subject: [PATCH 1/2] Explain NULLs when teaching count

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.
---
 _episodes/06-agg.md | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/_episodes/06-agg.md b/_episodes/06-agg.md
index 4a753759..58ba3818 100644
--- a/_episodes/06-agg.md
+++ b/_episodes/06-agg.md
@@ -95,11 +95,15 @@ 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.
+Even a column other than `reading` could be used,
+but note that any `NULL` value will not be counted
+(try counting the `person` column instead).
+This perhaps non-obvious behavior
+of aggregation functions is covered later
+in this episode.
 
 SQL lets us do several aggregations at once.
 We can,

From 7fff12b8e358bb004a724e62b5375a0e35082db7 Mon Sep 17 00:00:00 2001
From: Chris Roat <1053153+chrisroat@users.noreply.github.com>
Date: Mon, 14 Sep 2020 21:49:44 -0700
Subject: [PATCH 2/2] Clarify how count works - counts rows, not values - be
 more specific about the NULL in the person column of the dataset

---
 _episodes/06-agg.md | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/_episodes/06-agg.md b/_episodes/06-agg.md
index 58ba3818..69f37bba 100644
--- a/_episodes/06-agg.md
+++ b/_episodes/06-agg.md
@@ -97,10 +97,11 @@ SELECT sum(reading) FROM Survey WHERE quant = 'sal';
 We used `count(reading)` here,
 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
-(try counting the `person` column instead).
+(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.