def get_value(key):
    cursor.execute('SELECT value FROM kv_store WHERE key = ?', (key,))
    result = cursor.fetchone()
    return result[0] if result else None

# 示例用法
print(get_value('name'))  # 输出: John Doe
print(get_value('phone'))  # 输出: None

def get_all_items():
    cursor.execute('SELECT key, value FROM kv_store')
    return dict(cursor.fetchall())

# 示例用法
print(get_all_items())
# 输出: {'name': 'John Doe', 'email': 'john@example.com'}

class KeyValueStore:
    def __init__(self, db_file=':memory:'):
        self.conn = sqlite3.connect(db_file)
        self.cursor = self.conn.cursor()
        self._create_table()

    def _create_table(self):
        self.cursor.execute('''
        CREATE TABLE IF NOT EXISTS kv_store (
            key TEXT PRIMARY KEY,
            value TEXT
        )
        ''')
        self.conn.commit()

    def __setitem__(self, key, value):
        self.cursor.execute('''
        INSERT OR REPLACE INTO kv_store (key, value)
        VALUES (?, ?)
        ''', (key, str(value)))
        self.conn.commit()

    def __getitem__(self, key):
        self.cursor.execute('SELECT value FROM kv_store WHERE key = ?', (key,))
        result = self.cursor.fetchone()
        if result:
            return result[0]
        raise KeyError(key)

    def __delitem__(self, key):
        self.cursor.execute('DELETE FROM kv_store WHERE key = ?', (key,))
        if self.cursor.rowcount == 0:
            raise KeyError(key)
        self.conn.commit()

    def __contains__(self, key):
        self.cursor.execute('SELECT 1 FROM kv_store WHERE key = ?', (key,))
        return self.cursor.fetchone() is not None

    def close(self):
        self.conn.close()

    # 上下文管理器支持
    def __enter__(self):
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        self.close()

# 使用示例
with KeyValueStore('my_store.db') as kv:
    kv['language'] = 'Python'
    kv['version'] = '3.9'
    print(kv['language'])  # 输出: Python
    del kv['version']

import json

class JSONKeyValueStore(KeyValueStore):
    def __setitem__(self, key, value):
        super().__setitem__(key, json.dumps(value))

    def __getitem__(self, key):
        return json.loads(super().__getitem__(key))

# 使用示例
with JSONKeyValueStore('json_store.db') as kv:
    kv['config'] = {'theme': 'dark', 'font_size': 14}
    print(kv['config']['theme'])  # 输出: dark