diff --git a/SQL/lessons/sql_3.md b/SQL/lessons/sql_3.md
index d2aca79..5f3705d 100644
--- a/SQL/lessons/sql_3.md
+++ b/SQL/lessons/sql_3.md
@@ -215,6 +215,19 @@ SELECT * FROM people;
 
 ### Locks
 
+```sql
+BEGIN;
+LOCK TABLE people IN ROW EXCLUSIVE MODE;
+SELECT * FROM people WHERE id = 12 FOR UPDATE;
+
+-- start a new session and run UPDATE people SET name = 'Matt' WHERE id = 12;
+-- switch back to original session
+
+UPDATE people SET age = 43 WHERE id = 12;
+SELECT * FROM people;
+END;
+```
+
 ### Privileges
 
 ### Denormalization