sqlalchemy

Khóa học miễn phí Using Query nhận dự án làm có lương

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SQLAlchemy ORM – Using Query

All SELECT statements generated by SQLAlchemy ORM are constructed by Query object. It provides a generative interface, hence successive calls return a new Query object, a copy of the former with additional criteria and options associated with it.

Query objects are initially generated using the query() method of the Session as follows −

q = session.query(mapped class)

Following statement is also equivalent to the above given statement −

q = Query(mappedClass, session)

The query object has all() method which returns a resultset in the form of list of objects. If we execute it on our customers table −

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

This statement is effectively equivalent to following SQL expression −

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

The result object can be traversed using For loop as below to obtain all records in underlying customers table. Here is the complete code to display all records in Customers table −

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 console shows list of records as below −

Name: Ravi Kumar Address: Station Road Nanded Email: ravi@gmail.com
Name: Komal Pande Address: Koti, Hyderabad Email: komal@gmail.com
Name: Rajender Nath Address: Sector 40, Gurgaon Email: nath@gmail.com
Name: S.M.Krishna Address: Budhwar Peth, Pune Email: smk@gmail.com

The Query object also has following useful methods −

Sr.No. Method & Description
1

add_columns()

It adds one or more column expressions to the list of result columns to be returned.
2

add_entity()

It adds a mapped entity to the list of result columns to be returned.
3

count()

It returns a count of rows this Query would return.
4

delete()

It performs a bulk delete query. Deletes rows matched by this query from the database.
5

distinct()

It applies a DISTINCT clause to the query and return the newly resulting Query.
6

filter()

It applies the given filtering criterion to a copy of this Query, using SQL expressions.
7

first()

It returns the first result of this Query or None if the result doesn’t contain any row.
8

get()

It returns an instance based on the given primary key identifier providing direct access to the identity map of the owning Session.
9

group_by()

It applies one or more GROUP BY criterion to the query and return the newly resulting Query
10

join()

It creates a SQL JOIN against this Query object’s criterion and apply generatively, returning the newly resulting Query.
11

one()

It returns exactly one result or raise an exception.
12

order_by()

It applies one or more ORDER BY criterion to the query and returns the newly resulting Query.
13

update()

It performs a bulk update query and updates rows matched by this query in the database.

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