外键及其四种约束讲解
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#encoding: utf-8 from sqlalchemy import create_engine,Column,Integer,String,Float,func,and_,or_,Text, ForeignKey from sqlalchemy.ext.declarative import declarative_base from sqlalchemy.orm import sessionmaker from random import randint HOSTNAME = ‘127.0.0.1‘ PORT = 3306 DATABASE = ‘first_sqlalchemy‘ USERNAME = ‘root‘ PASSWORD = ‘123456‘ #dialect+driver://username:[email protected]: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) #父表/从表 #user/article class User(Base): __tablename__ = ‘user‘ id = Column(Integer,primary_key=True,autoincrement=True) username = Column(String(32),nullable=False) class Article(Base): __tablename__ = ‘article‘ id = Column(Integer,primary_key=True,autoincrement=True) title = Column(String(50),nullable=False) content = Column(Text,nullable=False) #引用主表的外键一定要跟主表的数据类型相同 #restrict 本意是拒绝的意思,no action 也是,从表(子表)关联了主表, #关联字段 ondelect = ‘RESTRICT‘ 或 ondelect = ‘NO ACTION‘,主表删除会报错, #不允许删除 #如果 设置 ondelect = "CASCADE",主表删除,子表也会随之删除 #如果 设置 ondelect = "SET NULL",主表删除,子表的关联字段会表设置null #也就是uid = null ,还有一种情况就是 nullable 不能设置成False uid = Column(Integer,ForeignKey(‘user.id‘,ondelete=‘RESTRICT‘)) Base.metadata.drop_all() Base.metadata.create_all() # 查看 表创建的sql 语句 # show create table articleG; ‘‘‘ Create Table: CREATE TABLE `article` ( `id` int(11) NOT NULL AUTO_INCREMENT, `title` varchar(50) NOT NULL, `content` text NOT NULL, `uid` int(11) DEFAULT NULL, PRIMARY KEY (`id`), KEY `uid` (`uid`), CONSTRAINT `article_ibfk_1` FOREIGN KEY (`uid`) REFERENCES `user` (`id`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8 ‘‘‘ #添加数据 user = User(username = ‘xiaowu‘) session.add(user) session.commit() article = Article(title = ‘圣墟‘,content = ‘吹牛逼死坑的小说,天天吹水逼‘,uid = 1) session.add(article) session.commit() #删除一条记录的sql delete from user where id = 1;
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