Add Count The Number Of Trues In An Aggregate Query as a postgres til

postgres/count-the-number-of-trues-in-an-aggregate-query.md

@@ -0,0 +1,26 @@
+# Count The Number Of Trues In An Aggregate Query
+
+The `sum` function is an aggregate function that allows you to sum up a bunch
+of integers. What if you want to sum up a boolean column? You may want to know
+how many times `true` appears in a collection of grouped records.
+
+This can be done by mixing in a `case` statement.
+
+```sql
+select
+  author_id,
+  sum(case when available then 1 else 0 end)
+from books
+group by author_id;
+```
+
+Here, we are able to find out for each author how many books they have
+available.
+
+If we want to count `false` values, we can just invert the `sum` statement:
+
+```sql
+sum(case when available then 0 else 1 end)
+```
+
+[source](https://stackoverflow.com/a/5396728/535590)