Add Group By The Result Of A Function Call as a postgres til

README.md

@@ -9,7 +9,7 @@ and pairing with smart people at Hashrocket.
 
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-_866 TILs and counting..._
+_867 TILs and counting..._
 
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@@ -434,6 +434,7 @@ _866 TILs and counting..._
 - [Get The Size Of A Table](postgres/get-the-size-of-a-table.md)
 - [Get The Size Of An Index](postgres/get-the-size-of-an-index.md)
 - [Getting A Slice Of An Array](postgres/getting-a-slice-of-an-array.md)
+- [Group By The Result Of A Function Call](postgres/group-by-the-result-of-a-function-call.md)
 - [Insert Just The Defaults](postgres/insert-just-the-defaults.md)
 - [Integers In Postgres](postgres/integers-in-postgres.md)
 - [Intervals Of Time By Week](postgres/intervals-of-time-by-week.md)

postgres/group-by-the-result-of-a-function-call.md

@@ -0,0 +1,31 @@
+# Group By The Result Of A Function Call
+
+Typically, a query that I write involving a `group by` will look more or less
+like this:
+
+```sql
+select category, count(*)
+  from products
+  group by category;
+```
+
+The `category` column is the thing I'm grouping by. In this case, I'm doing a
+little data exploration.
+
+We are not strictly limited to grouping by a column. We can use all sorts of
+functions offered by Postgres to get at more interesting results. [String
+functions](https://www.postgresql.org/docs/current/functions-string.html) are a
+great place to start.
+
+Let's say our `products` table also has an `identifier` column with a naming
+scheme where the first three letters of the identifier correspond to the
+product's classification. We can group by that part of the `identifier`:
+
+```sql
+select substring(identifier from 1 for 3), count(*)
+  from products
+  group by substring(identifier from 1 for 3);
+```
+
+The funkiness of the `substring` syntax aside, we were able to group our
+products in a new way and learn something about our data.