input: Write a python function to retrieve all user names from the database table.
  assert retrieve_user_names() == ['John', 'Thomas', 'Emily']
output: def retrieve_user_names():
      import sqlite3
      
      # Assuming the database file is 'database.db' and the table name is 'users'
      conn = sqlite3.connect('database.db')
      cursor = conn.cursor()
      
      cursor.execute("SELECT name FROM users")
      rows = cursor.fetchall()
      
      user_names = [row[0] for row in rows]
      
      conn.close()
      
      return user_names