--- a/doc/book/en/annexes/rql/language.rst	Fri Jun 17 09:37:49 2011 +0200
+++ b/doc/book/en/annexes/rql/language.rst	Thu Jun 16 12:31:27 2011 +0200
@@ -152,6 +152,10 @@
 
 - Aggregate Functions: COUNT, MIN, MAX, AVG, SUM, GROUP_CONCAT
 
+.. note::
+   Aggregate functions will return None if there is no result row.
+
+
 Having
 ```````