SQLAlchemy ORM – 使用查询

SQLAlchemy ORM 生成的所有 SELECT 语句都是由 Query 对象构造的。它提供了一个生成接口，因此连续调用会返回一个新的 Query 对象，即前者的副本，并带有附加条件和与之相关的选项。

查询对象最初是使用 Session 的 query() 方法生成的，如下所示 –

q = session.query(mapped class)

以下语句也等效于上面给出的语句 –

q = Query(mappedClass, session)

查询对象具有 all() 方法，该方法以对象列表的形式返回结果集。如果我们在客户表上执行它 –

result = session.query(Customers).all()

此语句实际上等效于以下 SQL 表达式 –

SELECT customers.id 
AS customers_id, customers.name 
AS customers_name, customers.address 
AS customers_address, customers.email 
AS customers_email
FROM customers

可以使用 For 循环遍历结果对象，如下所示，以获取基础客户表中的所有记录。这是在客户表中显示所有记录的完整代码 –

from sqlalchemy import Column, Integer, String
from sqlalchemy import create_engine
engine = create_engine('sqlite:///sales.db', echo = True)
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()

class Customers(Base):
   __tablename__ = 'customers'
   id = Column(Integer, primary_key =  True)
   name = Column(String)

   address = Column(String)
   email = Column(String)

from sqlalchemy.orm import sessionmaker
Session = sessionmaker(bind = engine)
session = Session()
result = session.query(Customers).all()

for row in result:
   print ("Name: ",row.name, "Address:",row.address, "Email:",row.email)

Python 控制台显示如下记录列表 –

Name: Ravi Kumar Address: Station Road Nanded Email: [email protected]
Name: Komal Pande Address: Koti, Hyderabad Email: [email protected]
Name: Rajender Nath Address: Sector 40, Gurgaon Email: [email protected]
Name: S.M.Krishna Address: Budhwar Peth, Pune Email: [email protected]

Query 对象还有以下有用的方法 –