input: Write a python function to retrieve data from an SQLite database.
  assert retrieve_data('data.db') == [(1, 'value1'), (2, 'value2')]
  assert retrieve_data('test.db') == [(3, 'value3'), (4, 'value4')]
  assert retrieve_data('empty.db') == []
output: def retrieve_data(db_name):
      import sqlite3
  
      conn = sqlite3.connect(db_name)
      cursor = conn.cursor()
      cursor.execute("SELECT * FROM my_table")
      data = cursor.fetchall()
      conn.close()
      
      return data