如何修复节点parent_id？

import sqlite3

# 假设我们有一个SQLite数据库
conn = sqlite3.connect('example.db')
cursor = conn.cursor()

# 创建示例表
cursor.execute('''CREATE TABLE IF NOT EXISTS nodes (
                    id INTEGER PRIMARY KEY,
                    name TEXT NOT NULL,
                    parent_id INTEGER)''')

# 插入示例数据
cursor.execute("INSERT INTO nodes (name, parent_id) VALUES ('Node1', NULL)")
cursor.execute("INSERT INTO nodes (name, parent_id) VALUES ('Node2', 1)")
cursor.execute("INSERT INTO nodes (name, parent_id) VALUES ('Node3', 999)")  # 孤立节点
cursor.execute("INSERT INTO nodes (name, parent_id) VALUES ('Node4', 2)")
cursor.execute("INSERT INTO nodes (name, parent_id) VALUES ('Node5', 4)")  # 循环引用

conn.commit()

def find_invalid_nodes():
    cursor.execute("SELECT id, parent_id FROM nodes WHERE parent_id NOT IN (SELECT id FROM nodes) OR id = parent_id")
    return cursor.fetchall()

def fix_invalid_nodes():
    invalid_nodes = find_invalid_nodes()
    for node in invalid_nodes:
        node_id, parent_id = node
        if parent_id not in [n[0] for n in cursor.execute("SELECT id FROM nodes")]:
            print(f"Fixing isolated node {node_id}: Setting parent_id to NULL")
            cursor.execute("UPDATE nodes SET parent_id = NULL WHERE id = ?", (node_id,))
        elif node_id == parent_id:
            print(f"Fixing self-referencing node {node_id}: Setting parent_id to NULL")
            cursor.execute("UPDATE nodes SET parent_id = NULL WHERE id = ?", (node_id,))
    conn.commit()

fix_invalid_nodes()