通过用户模型，对数据库进行增删改查操作

#增
user = User(username=‘mis1234‘,password=‘111111‘)
db.session.add(user)
db.session.commit()

#查
user = User.query.filter(User.username == ‘sk‘).first()
print(user.id,user.password)

#改
user = User.query.filter(User.username == ‘mis1234‘).first()
user.password=‘888888‘
user.username=‘sk‘
db.session.commit()
print(user.id,user.password)

#删
user = User.query.filter(User.username==‘sk‘).first()
db.session.delete(user)
db.session.commit()