SQLAlchemy Core – Delete Statement

In this article, we are going to see how to use the DELETE statement in SQLAlchemy against a PostgreSQL database in python.

Creating table for demonstration:

Import necessary functions from the SQLAlchemy package. Establish connection with the PostgreSQL database using create_engine() function as shown below, create a table called books with columns book_id and book_price. Insert record into the tables using insert() and values() function as shown.

Python3

# import necessary packages
from sqlalchemy.engine import result
import sqlalchemy
from sqlalchemy import create_engine, MetaData,\
Table, Column, Numeric, Integer, VARCHAR, update, delete

# establish connections
engine = create_engine(
    "database+dialect://username:password0@host:port/databasename")

# initialize the Metadata Object
meta = MetaData(bind=engine)
MetaData.reflect(meta)

# create a table schema
books = Table(
    'books', meta,
    Column('book_id', Integer, primary_key=True),
    Column('book_price', Numeric),
    Column('genre', VARCHAR),
    Column('book_name', VARCHAR)
)

meta.create_all(engine)

# insert records into the table
statement1 = books.insert().values(book_id=1, book_price=12.2,
                                   genre='fiction',
                                   book_name='Old age')
statement2 = books.insert().values(book_id=2, book_price=13.2,
                                   genre='non-fiction',
                                   book_name='Saturn rings')
statement3 = books.insert().values(book_id=3, book_price=121.6,
                                   genre='fiction',
                                   book_name='Supernova')
statement4 = books.insert().values(book_id=4, book_price=100,
                                   genre='non-fiction',
                                   book_name='History of the world')
statement5 = books.insert().values(book_id=5, book_price=1112.2,
                                   genre='fiction',
                                   book_name='Sun city')

# execute the insert records statement
engine.execute(statement1)
engine.execute(statement2)
engine.execute(statement3)
engine.execute(statement4)
engine.execute(statement5)

Output:

Sample table

Delete table elements in SQLAlchemy

Deleting table elements have a slightly different procedure than that of a conventional SQL query which is  shown below

from sqlalchemy import delete
Tablename.delete().where(Tablename.c.column_name == value)

Get the books table from the Metadata object initialized while connecting to the database. Pass the delete query to the execute() function and get all the results using fetchall() function. Use a for loop to iterate through the results.

The SQLAlchemy query shown in the below code deletes the “non-fiction” genre this will effectively delete multiple rows at one go. Then, we can write a conventional SQL query and use fetchall() to print the results to check whether the table is updated properly.

Python3

# delete
dele = BOOKS.delete().where(BOOKS.c.genre == "non-fiction")

engine.execute(dele)

# write the SQL query inside the
# text() block to fetch all records
sql = text("SELECT * from BOOKS")

# Fetch all the records
result = engine.execute(sql).fetchall()

# View the records
for record in result:
    print("\n", record)

Output:

The output of delete function