In this article, we are going to see how to use the UPDATE 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 packagesfrom sqlalchemy.engine import resultimport sqlalchemyfrom sqlalchemy import create_engine, MetaData,Table, Column, Numeric, Integer, VARCHAR, update# establish connectionsengine = create_engine(# initialize the Metadata Objectmeta = MetaData(bind=engine)MetaData.reflect(meta)# create a table schemabooks = 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 tablestatement1 = 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 statementengine.execute(statement1)engine.execute(statement2)engine.execute(statement3)engine.execute(statement4)engine.execute(statement5) |
Output:
Sample table
Implementing a query to update table elements in SQLAlchemy
Example 1: Query to update table
Updating table elements have a slightly different procedure than that of a conventional SQL query which is shown below
from sqlalchemy import update
upd = update(tablename)
val = upd.values({"column_name":"value"})
cond = val.where(tablename.c.column_name == value)
Get the books table from the Metadata object initialized while connecting to the database. Pass the update 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 updates the book name of row 3 as “2022 future ahead”. Then, we can write a conventional SQL query and use fetchall() to print the results to check whether the table is updated properly.
Python3
# Get the `books` table from the Metadata objectBOOKS = meta.tables['books']# updateu = update(BOOKS)u = u.values({"book_name": "2022 future ahead"})u = u.where(BOOKS.c.book_id == 3)engine.execute(u)# write the SQL query inside the# text() block to fetch all recordssql = text("SELECT * from BOOKS")# Fetch all the recordsresult = engine.execute(sql).fetchall()# View the recordsfor record in result: print("\n", record) |
Output:
The result of the Update query.
Example 2: Query to update a table based on the value
Let us see another example related to update query. The update query shown below updates the genre fiction as “sci-fi”.
Tablename.update().where(Tablename.c.column_name == ‘value’).values(column_name = ‘value’)
Python3
# Get the `books` table from the Metadata objectBOOKS = meta.tables['books']# updatestmt = BOOKS.update().where(BOOKS.c.genre == 'non-fiction' ).values(genre = 'sci-fi')engine.execute(stmt)# write the SQL query inside the# text() block to fetch all recordssql = text("SELECT * from BOOKS")# Fetch all the recordsresult = engine.execute(sql).fetchall()# View the recordsfor record in result: print("\n", record) |
Output:
The output of the update query
Example 3: Query to update a table based on the Condition
The below query updates the book_price by adding 50 bucks to books amounting to less than or equal to100.
Python3
# Get the `books` table from the Metadata objectBOOKS = meta.tables['books']# updatestmt = BOOKS.update().where(BOOKS.c.book_price <= 100 ).values(book_price= BOOKS.c.book_price + 50)engine.execute(stmt)# write the SQL query inside the# text() block to fetch all recordssql = text("SELECT * from BOOKS")# Fetch all the recordsresult = engine.execute(sql).fetchall()# View the recordsfor record in result: print("\n", record) |
Output:
The output of update query
