In this article, we will see how to select the count of rows using SQLAlchemy in Python.
Since we are going to use MySQL in this post, we will also install a SQL connector for MySQL in Python. However, none of the code implementations changes with change in the database except for the SQL connectors.
pip install pymysql
We will use the sample sakila database from mySQL. In this article, we will cover 2 examples, one each for SQLAchemy Core and ORM layers. In both examples, we will count the number of records present in the actor table within the sakila database. If you do not have the sakila database and want to follow along with this article without installing it then use the SQL script present in the link mentioned below to create the required schema and actor table along with the records. Sakila Actor Table Script
The SQL query which we are looking at in the below two examples is –
SELECT COUNT(*) FROM sakila.`actor`;
SQLAlchemy Core
SQLAlchemy Core is a schema-centric model that means everything is treated as a part of the database i.e., rows, columns, tables, etc. In the above example, we have created the metadata object to access the database objects like the table. Using this object we get the metadata of the actor table. This metadata information is then used to query to the table using the SQLAlchemy syntax mentioned below.
Syntax: sqlalchemy.select([sqlalchemy.func.count()]).select_from(sqlalchemy.DeclarativeMeta).scalar()
Python
import sqlalchemy as db# DEFINE THE ENGINE (CONNECTION OBJECT)# CREATE THE METADATA OBJECT TO ACCESS THE TABLEmeta_data = db.MetaData(bind=engine)db.MetaData.reflect(meta_data)# GET THE `actor` TABLE FROM THE METADATA OBJECTactor_table = meta_data.tables['actor']# SELECT COUNT(*) FROM Actorresult = db.select([db.func.count()]).select_from(actor_table).scalar()print("Count:", result) |
Output:
Count: 200
SQLAlchemy ORM
SQLAlchemy Core uses an object-centric view that encapsulates the schema with business objects. It is a more pythonic implementation as we can see the tables represented in the class format. We have used this class object to query the actor table using the SQLAlchemy syntax mentioned below.
Syntax: sqlalchemy.orm.Session.query(sqlalchemy.DeclarativeMeta).count()
Python
import sqlalchemy as dbfrom sqlalchemy.ext.declarative import declarative_baseBase = declarative_base()# DEFINE THE ENGINE (CONNECTION OBJECT)# CREATE THE TABLE MODEL TO USE IT FOR QUERYINGclass Actor(Base): __tablename__ = 'actor' actor_id = db.Column(db.SmallInteger, primary_key=True, autoincrement=True) first_name = db.Column(db.String(45)) last_name = db.Column(db.String(45)) last_update = db.Column(db.DateTime) city = db.Column(db.String(20))# CREATE A SESSION OBJECT TO INITIATE QUERY IN DATABASEfrom sqlalchemy.orm import sessionmakerSession = sessionmaker(bind = engine)session = Session()# SELECT COUNT(*) FROM Actorresult = session.query(Actor).count()print("Count:", result) |
Output:
Count: 200
