【Flask】Sqlalchemy limit, offset slice操作

### limit、offset和切片操作：
1. limit：可以限制每次查询的时候只查询几条数据。
2. offset：可以限制查找数据的时候过滤掉前面多少条。
3. 切片：可以对Query对象使用切片操作，来获取想要的数据。可以使用`slice(start,stop)`方法来做切片操作。也可以使用`[start:stop]`的方式来进行切片操作。一般在实际开发中，中括号的形式是用得比较多的。示例代码如下：

 # coding:utf-8

 from datetime import datetime
 from sqlalchemy import create_engine, Column, Integer, String, DateTime

 from sqlalchemy.ext.declarative import declarative_base
 from sqlalchemy.orm import sessionmaker, relationship, backref

 HOSTNAME = '127.0.0.1'
 PORT = 3306
 DATABASE = 'sqlalchemy_first'
 USERNAME = 'root'
 PASSWORD = ''

 DB_URI = 'mysql+pymysql://{username}:{password}@{host}:{port}/{dbname}?charset=utf8'.format(
         username=USERNAME, password=PASSWORD, host=HOSTNAME, port=PORT, dbname=DATABASE
 )
 engine = create_engine(DB_URI)
 Base = declarative_base(engine)
 Session = sessionmaker(engine)
 session = Session()

 class Article(Base):
     __tablename__ = 'article'
     id = Column(Integer, primary_key=True, autoincrement=True)
     title = Column(String(50), nullable=False)
     createtime = Column(DateTime, nullable=False, default=datetime.now)

     def __repr__(self):
         return "Article<title:{}>".format(self.title)

 Base.metadata.drop_all()
 Base.metadata.create_all()

 for _ in xrange(100):
     article = Article(title='article{}'.format(_))
     session.add(article)
 session.commit()

 # limit
 articles = session.query(Article).limit(10).all()
 print articles

 # offset
 articles = session.query(Article).offset(10).limit(10).all()
 print articles

 # slice, sliece(start, end) 从start取到end.
 articles = session.query(Article).slice(10, 20).all()
 print articles

 # 切片
 articles = session.query(Article)[0:10]
 print articles