我如何在sequelize中编写这个查询？

const Sequelize = require('sequelize');
const sequelize = new Sequelize('database', 'username', 'password', {
  host: 'localhost',
  dialect: 'mysql',
});

const User = sequelize.define('User', {
  firstName: {
    type: Sequelize.STRING,
    allowNull: false
  },
  lastName: {
    type: Sequelize.STRING,
    allowNull: false
  },
  age: {
    type: Sequelize.INTEGER,
    allowNull: false
  }
});

// 查询所有用户
User.findAll().then(users => {
  console.log(users);
});

// 查询指定条件的用户
User.findAll({
  where: {
    age: {
      [Sequelize.Op.gt]: 18
    }
  }
}).then(users => {
  console.log(users);
});

// 查询单个用户
User.findOne({
  where: {
    firstName: 'John'
  }
}).then(user => {
  console.log(user);
});