diff --git a/README.md b/README.md
index 66b2944..460df2f 100644
--- a/README.md
+++ b/README.md
@@ -10,4 +10,6 @@ WITH RECURSIVE hierarcy (id, title, description, parent_id) AS
       ON hierarcy.parent_id = items.id
 )
 SELECT * FROM hierarcy;
-```
\ No newline at end of file
+```
+
+https://www.mysqltutorial.org/mysql-adjacency-list-tree/
\ No newline at end of file