Update sqlite3/README.md

sqlite3/README.md

@@ -53,6 +53,15 @@ class DB(sqlite3.Connection):
         return [row[0] for row in result]
 
     def __repr__(self): return repr(self.keys())
+
+    def exists(self, key):
+        with self:
+            cur = self.execute('''
+                SELECT 1 FROM kv_store
+                WHERE key = ?
+                LIMIT 1
+            ''', (key,))
+            return bool(cur.fetchone())
 ```
 # Usage
 ```python
@@ -75,4 +84,9 @@ complex_obj = {'name': 'John', 'age': 30, 'city': 'New York'}
 db['complex'] = complex_obj
 loaded_obj = db['complex']
 print(loaded_obj)  # 출력: {'name': 'John', 'age': 30, 'city': 'New York'} (dict 타입)
+
+db['test_key'] = 'test_value'
+
+print(db.exists('test_key'))  # True
+print(db.exists('non_existent_key'))  # False
 ```