input: Write a python function to create a new item and retrieve all items from the database using Flask and SQLAlchemy.
  assert add_item({'name': 'Book', 'description': 'A good book'})['name'] == 'Book'
  assert add_item({'name': 'Pen', 'description': 'A blue pen'})['description'] == 'A blue pen'
  assert len(get_items()) >= 2
output: from flask import Flask, request, jsonify
  from flask_sqlalchemy import SQLAlchemy
  
  app = Flask(__name__)
  app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///test.db'
  db = SQLAlchemy(app)
  
  class Item(db.Model):
      id = db.Column(db.Integer, primary_key=True)
      name = db.Column(db.String(80), nullable=False)
      description = db.Column(db.String(120), nullable=False)
  
  db.create_all()
  
  def add_item(data):
      new_item = Item(name=data['name'], description=data['description'])
      db.session.add(new_item)
      db.session.commit()
      return {'id': new_item.id, 'name': new_item.name, 'description': new_item.description}
  
  def get_items():
      items = Item.query.all()
      return [{'id': item.id, 'name': item.name, 'description': item.description} for item in items]