Python：sqlalchemy连接数据库

安装：

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

举例：（在url后面加入?charset=utf8可以防止乱码）

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

create_engine接受一个url，格式为：

# '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

---

简单demo：

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
'''

---

将查询结果映射为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
'''

---

查询：

session的query方法除了可以接受Base子类对象作为参数外，还可以是字段，如：

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：

# = / 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'))

Query的方法：

all()方法以列表形式返回结果集：

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',)]

first()方法返回单个结果。（若结果集为空则返回None）

print(queryObject.first()) 　#　('ns',)

one()方法返回单个结果，与first()方法不同的是：当结果集中没有元素或有多于一个元素会抛出异常。
one_or_none()方法同one()一样，不同是结果集为空则返回None，为多个抛出异常。

查询数量：

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

分组：

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

---

嵌套SQL语句：

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)]