标签：rom 功能 代码 mysq utf-8 drop key group left join
### join：
1. join分为left join（左外连接）和right join（右外连接）以及内连接（等值连接）。
2. 参考的网页：http://www.jb51.net/article/15386.htm
3. 在sqlalchemy中，使用join来完成内连接。在写join的时候，如果不写join的条件，那么默认将使用外键来作为条件连接。
4. query查找出来什么值，不会取决于join后面的东西，而是取决于query方法中传了什么参数。就跟原生sql中的select 后面那一个一样。
比如现在要实现一个功能，要查找所有用户，按照发表文章的数量来进行排序。示例代码如下：
1 # coding:utf-8 2 # Author: liangjun.chen 3 4 from datetime import datetime 5 from sqlalchemy import create_engine, Column, Integer, String, DateTime, Text, ForeignKey, func 6 7 from sqlalchemy.ext.declarative import declarative_base 8 from sqlalchemy.orm import sessionmaker, relationship, backref 9 10 HOSTNAME = ‘127.0.0.1‘ 11 PORT = 3306 12 DATABASE = ‘sqlalchemy_first‘ 13 USERNAME = ‘root‘ 14 PASSWORD = ‘123456‘ 15 16 DB_URI = ‘mysql+pymysql://{username}:{password}@{host}:{port}/{dbname}?charset=utf8‘.format( 17 username=USERNAME, password=PASSWORD, host=HOSTNAME, port=PORT, dbname=DATABASE 18 ) 19 engine = create_engine(DB_URI) 20 Base = declarative_base(engine) 21 Session = sessionmaker(engine) 22 session = Session() 23 24 25 # article 26 class Article(Base): 27 __tablename__ = ‘article‘ 28 id = Column(Integer, primary_key=True, autoincrement=True) 29 title = Column(String(50), nullable=False) 30 create_time = Column(DateTime,nullable=False, default=datetime.now) 31 uid = Column(Integer, ForeignKey("user.id")) 32 33 # relation 正向引用, backref反向引用 34 author = relationship("User", backref=backref(‘articles‘, lazy=‘dynamic‘)) 35 36 def __repr__(self): 37 return "Article <title: {}>".format(self.title) 38 39 40 class User(Base): 41 __tablename__ = ‘user‘ 42 id = Column(Integer, primary_key=True, autoincrement=True) 43 username = Column(String(50), nullable=False) 44 45 46 # Base.metadata.drop_all() 47 # Base.metadata.create_all() 48 # 49 # user1 = User(username=‘saber‘) 50 # user2 = User(username=‘slamdunk‘) 51 # 52 # 53 # for _ in xrange(1): 54 # article = Article(title=‘article{}‘.format(_)) 55 # article.author = user1 56 # session.add(article) 57 # session.commit() 58 # 59 # for _ in xrange(3): 60 # article = Article(title=‘article{}‘.format(_)) 61 # article.author = user2 62 # session.add(article) 63 # session.commit() 64 65 # 找到所有用户, 按照发表文章数量进行排序 66 67 result = session.query(User.username, func.count(Article.id)).join(Article, Article.uid==User.id).group_by(User.id).order_by(func.count(Article.id).desc()).all() 68 print result
标签：rom 功能 代码 mysq utf-8 drop key group left join
原文地址：https://www.cnblogs.com/chen0427/p/8799237.html
