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

README.md

@@ -10,7 +10,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 For a steady stream of TILs from a variety of rocketeers, checkout
 [til.hashrocket.com](https://til.hashrocket.com/).
 
-_853 TILs and counting..._
+_854 TILs and counting..._
 
 ---
 
@@ -403,6 +403,7 @@ _853 TILs and counting..._
 - [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 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)
 - [Create An Index Without Locking The Table](postgres/create-an-index-without-locking-the-table.md)
 - [Create hstore From Two Arrays](postgres/create-hstore-from-two-arrays.md)

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)