limit和offset、切片操作

#encoding: utf-8

from sqlalchemy import create_engine,Column,Integer,String,Float,func,and_,or_,\
    DateTime
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker
from random import randint
from datetime import datetime

HOSTNAME = '127.0.0.1'

PORT = 3306

DATABASE = 'first_sqlalchemy'

USERNAME = 'root'

PASSWORD = '123456'

#dialect+driver://username:password@host:port/database
DB_URI = "mysql+pymysql://{username}:{password}@{host}:{port}/" \
         "{db}?charset=utf8".format(username=USERNAME,password=PASSWORD,host=HOSTNAME,port=PORT,db=DATABASE)

engine = create_engine(DB_URI)

Base = declarative_base(engine)

# Session = sessionmaker(engine)
# session = Session()

session = sessionmaker(engine)() #Session(**local_kw)


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

    def __repr__(self):
        return '<article:{title}>'.format(title=self.title)


# Base.metadata.drop_all()
#
# Base.metadata.create_all()
#
#
# for x in range(0,100):
#     article = Article(title = 'title%s'%x)
#     session.add(article)
# session.commit()


#第一limit的用法，限制查詢多少數據
article = session.query(Article).limit(10).all()#用limit限制只查詢10個數據
print(article)

#第二個參數offset的用法，本意是偏移量，在這里就是從多少開始查詢
article_offset = session.query(Article).offset(10).all()
print(article_offset)

#offset和limit聯合起來用，就相當於python 的字符串和列表、元祖的切片操作
article_offset_limit = session.query(Article).offset(10).limit(5).all()
print(article_offset_limit)

#如果查詢最新的10篇文章，就可以用order_by 和 limit 一起用
article_order_by_limit = session.query(Article).order_by(Article.id.desc()).limit(10).all()
print(article_order_by_limit)

#slice,本身就是切片的意思
article_order_by_slice = session.query(Article).order_by(Article.id.desc()).slice(0,10).all()
print(article_order_by_slice)

#還有一個更簡單的方法，就想python的列表切片操作

article_list_slice = session.query(Article).order_by(Article.id.desc())[0:10]
print(article_list_slice)