Python：sqlalchemy鏈接數據庫

安裝：

pip install sqlalchemy
# 安裝數據庫驅動：
pip install pymysql
pip install cx_oracle

舉例：（在url後面加入?charset=utf8能夠防止亂碼）

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

create_engine接受一個url，格式爲：

# '數據庫類型+數據庫驅動名稱://用戶名:口令@機器地址:端口號/數據庫名'
# 經常使用的
engine = create_engine('sqlite:///:memory:', echo=True)     # sqlite內存
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就能夠用tns鏈接

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簡單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語句等調試信息
session_maker = sessionmaker(bind=engine)
session = session_maker()

Base = declarative_base()
#對應一張表
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表則不作，不存在則建立。
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 '''

---

將查詢結果映射爲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 '''

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查詢：

session的query方法除了能夠接受Base子類對象做爲參數外，還能夠是字段，如：

query = session.query(Student.name, Student.age) # query爲一個sqlalchemy.orm.query.Query對象
for stu_name, stu_age in query:
    print(stu_name, stu_age)

查詢條件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結果集：

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()方法返回單個結果。（若結果集爲空則返回None）

print(queryObject.first()) 　#　('ns',)

one()方法返回單個結果，與first()方法不一樣的是：當結果集中沒有元素或有多於一個元素會拋出異常。
one_or_none()方法同one()同樣，不一樣是結果集爲空則返回None，爲多個拋出異常。

查詢數量：

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

分組：

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

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嵌套SQL語句：

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)]