Python SQLAlchemy数据库连接与查询操作详解

环境安装与驱动配置

pip install sqlalchemy
# 安装数据库驱动：
pip install pymysql
pip install cx_oracle

数据库引擎创建与连接

from sqlalchemy import create_engine
engine=create_engine('mysql+pymysql://username:password@hostname:port/dbname', echo=True) #echo=True 打印sql语句信息

常见数据库URL格式示例

# '数据库类型+数据库驱动名称://用户名:口令@机器地址:端口号/数据库名'
# 常用的
engine = create_engine('sqlite:///:memory:', echo=True)   # sqlite内存
engine = create_engine('sqlite:///./cnblogblog.db',echo=True) # sqlite文件
engine = create_engine("mysql+pymysql://username:password@hostname:port/dbname",echo=True) # mysql+pymysql
engine = create_engine('mssql+pymssql://username:password@hostname:port/dbname',echo=True) # mssql+pymssql
engine = create_engine('postgresql://scott:tiger@hostname:5432/dbname') # postgresql示例
engine = create_engine('oracle://scott:tiger@hostname:1521/sidname') # oracle
engine = create_engine('oracle+cx_oracle://scott:tiger@tnsname') #pdb就可以用tns连接

模型定义与Session会话管理

from sqlalchemy import create_engine, Column, Integer, String
from sqlalchemy.orm import sessionmaker
from sqlalchemy.ext.declarative import declarative_base 
engine = create_engine('oracle://spark:a@orclpdb',echo=True) #echo要求打印sql语句等调试信息
session_maker = sessionmaker(bind=engine)
session = session_maker()
Base = declarative_base()
#对应一张表
class Student(Base): 
  __tablename__ = 'STUDENT'
  id = Column('STUID', Integer, primary_key=True)
  name = Column('STUNAME', String(32), nullable=False)
  age = Column('STUAGE', Integer)
  def __repr__(self):
    return '<Student(id:%s, name:%s, age:%s)>' % (self.id, self.name, self.age)
Base.metadata.create_all(engine) #若存在STUDENT表则不做，不存在则创建。
queryObject = session.query(Student).order_by(Student.id.desc())
for ins in queryObject:
  print(ins.id, ins.name, ins.age)
'''
4 hey 24
3 lwtxxs 27
2 gyb 89
1 ns 23
'''

查询结果映射为DataFrame

import pandas as pd
df = pd.read_sql(session.query(Student).filter(Student.id > 1).statement, engine) 
print(df)
'''
  STUID STUNAME STUAGE
0   4   hey   24
1   2   gyb   89
2   3 lwtxxs   27
'''

字段查询与结果遍历

query = session.query(Student.name, Student.age) # query为一个sqlalchemy.orm.query.Query对象
for stu_name, stu_age in query:
  print(stu_name, stu_age)

查询条件Filter用法详解

# = / like
query.filter(Student.name == 'wendy')
query.filter(Student.name.like('%ed%'))
# in
query.filter(Student.name.in_(['wendy', 'jack']))
query.filter(Student.name.in_(
    session.query(User.name).filter(User.name.like('%ed%'))
))
# not in
query.filter(~Student.name.in_(['ed', 'wendy', 'jack']))
# is null / is not null
query.filter(Student.name == None)
query.filter(Student.name.is_(None))
query.filter(Student.name != None)
query.filter(Student.name.isnot(None))
# and
from sqlalchemy import and_, or_
query.filter(and_(Student.name == 'ed', Student.age != 23))
query.filter(Student.name == 'ed', Student.age != 23)
query.filter(Student.name == 'ed').filter(Student.age != 23)
# or
query.filter(or_(Student.name == 'ed', Student.name == 'wendy'))
# match
query.filter(Student.name.match('wendy'))

结果集获取方法：all与first

from sqlalchemy import or_, and_
queryObject = session.query(Student).filter(or_(Student.id == 1, Student.id == 2))
print(queryObject.all())  # [<Student(id:1, name:ns, age:23)>, <Student(id:2, name:gyb, age:89)>]
queryObject = session.query(Student.name).filter(or_(Student.id == 1, Student.id == 2))
print(queryObject.all())  # [('ns',), ('gyb',)]

精确结果获取：one与one_or_none

print(queryObject.first()) 　#　('ns',)

聚合统计与分组查询

from sqlalchemy import func
session.query(func.count(Student.id)).scalar()　# SELECT count("STUDENT"."STUID") AS count_1 FROM "STUDENT"

session.query(func.count(Student.id), Student.name).group_by(Student.name).all()

原生SQL语句执行

from sqlalchemy import text
query = session.query(Student.id, Student.name).filter(text('stuid>2'))
query = session.query('stuid', 'stuname', 'stuage').from_statement(\
text("select * from student where stuname=:stuname")).params(stuname='hey').all()  #[(4, 'hey', 24)]