Add Count How Many Records There Are Of Each Type as a postgres til

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
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-_1077 TILs and counting..._
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@@ -517,6 +517,7 @@ _1077 TILs and counting..._
 - [Configure The Timezone](postgres/configure-the-timezone.md)
 - [Constructing A Range Of Dates](postgres/constructing-a-range-of-dates.md)
 - [Convert A String To A Timestamp](postgres/convert-a-string-to-a-timestamp.md)
+- [Count How Many Records There Are Of Each Type](postgres/count-how-many-records-there-are-of-each-type.md)
 - [Count Records By Type](postgres/count-records-by-type.md)
 - [Count The Number Of Trues In An Aggregate Query](postgres/count-the-number-of-trues-in-an-aggregate-query.md)
 - [Create A Composite Primary Key](postgres/create-a-composite-primary-key.md)

postgres/count-how-many-records-there-are-of-each-type.md

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+# Count How Many Records There Are Of Each Type
+
+Let's say I have a `books` table full of data. One of the columns on this table
+is `status` which represents whether the book is published, in review, or still
+a draft.
+
+We can find out how many records (books) there are for each `status` using a
+`group by` clause and the `count` aggregate function.
+
+```sql
+> select status, count(*)
+    from books
+    group by status;
+
+  status   | count
+-----------+-------
+ ø         |   123
+ published |   611
+ draft     |   364
+ review    |   239
+(4 rows)
+```
+
+Because we don't have a `not null` constraint on the `status` column, there are
+also some records that have a null value.
+
+We can take this a step further by ordering the output in a consistent
+way—descending order of the count column.
+
+```sql
+> select status, count(*)
+    from books
+    group by status
+    order by 2 desc;
+
+  status   | count
+-----------+-------
+ published |   611
+ draft     |   364
+ review    |   239
+ ø         |   123
+(4 rows)
+```
+
+This `order by` clauses uses [a positional index from the select
+arguments](use-argument-indexes.md), so the `2` references the `count(*)`
+argument.