diff --git a/reference/mysql/functions/mysql-affected-rows.xml b/reference/mysql/functions/mysql-affected-rows.xml
index c78a8aced9..d38dec2866 100644
--- a/reference/mysql/functions/mysql-affected-rows.xml
+++ b/reference/mysql/functions/mysql-affected-rows.xml
@@ -50,6 +50,11 @@
    and then inserts the new record. This function returns the number of 
    deleted records plus the number of inserted records.
   </para> 
+  <para>
+   In the case of "INSERT ... ON DUPLICATE KEY UPDATE" queries, the 
+   return value will be <literal>1</literal> if an insert was performed, 
+   or <literal>2</literal> for an update of an existing row.   
+  </para>
  </refsect1>
 
  <refsect1 role="examples">