Update column in table using Python

This recipe lets you to perform simple update operations. If you wish to have finer control consider writing custom SQL queries in raw query recipe.

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Documentation

All required packages are automatically imported by MLJAR Studio for you so you don't have to worry about them.

# import packages
import psycopg
import os
from dotenv import load_dotenv
from psycopg import sql

Make sure you opened a connection in your notebook. To learn how to do it, check out open and test database connection example notebook.

# load credentials from .env file:
load_dotenv(override=True)

# get the credentials
def create_new_connection():
    try:
        conn = psycopg.connect(
            dbname=os.getenv("POSTGRES_DB_NAME"),
            user=os.getenv("POSTGRES_USERNAME"),
            password=os.getenv("POSTGRES_PASSWORD"),
            host=os.getenv("POSTGRES_HOST"),
            port=os.getenv("POSTGRES_PORT"),
        )
        return conn
    # check for errors
    except psycopg.Error as e:
        raise psycopg.Error(f"""
Error occurred while establishing connection: 
    {e}

Maybe you provided wrong credentials, use define new connection recipe to edit them.
Other option is that database server is down or you dont have the right acces to this database.
            """)

# open new connection:
conn = create_new_connection()

We will perform the update in table products. Lets list all columns of that table using show all columns recipe

# if connection was used and closed it is reopen here
if conn.closed:
    conn = create_new_connection()

# run query
with conn:
    with conn.cursor() as cur:

        # query db
        try:
            cur.execute("""
                    SELECT attname AS col, atttypid::regtype AS datatype
                    FROM pg_attribute
                    WHERE attrelid = %s::regclass 
                    AND attnum > 0
                    AND NOT attisdropped
                    ORDER BY attnum;""", 
                    ("products",))
        # check for errors
        except psycopg.ProgrammingError as e:
            raise psycopg.ProgrammingError(f"""
Problem running query:
    {e}

Did you spell everything correctly?
You can use show tables and columns recipes.
            """)

        # print the results
        print("Columns of products:")
        for column in cur.fetchall():
            print(f"{column}")

Lets check the contents of products table using select query recipe.

# if connection was used and closed it is reopen here
if conn.closed:
    conn = create_new_connection()

# run query
with conn:
    with conn.cursor() as cur:

        # query db
        try:
            cur.execute(
                sql.SQL("SELECT {columns} FROM {table}").format(
                    columns=sql.SQL("*"),
                    table=sql.Identifier("products"),
                )
            )
        # check for errors
        except psycopg.ProgrammingError as e:
            raise psycopg.ProgrammingError(f"""
Problem running query:
    {e}

Did you spell everything correctly?
You can use show tables and columns recipes.
            """)

        # print the results
        for row in cur.fetchall():
            print(f"{row}")

Lets increase price of every product by 15%. To do this we will use update column recipe. Checking show result will return results below the code.

# if connection was used and closed it is reopen here
if conn.closed:
    conn = create_new_connection()

# run query
with conn:
    with conn.cursor() as cur:

        # update data
        try:
            expression = sql.SQL("price * 1.15").format(sql.Placeholder())
            cur.execute(
                sql.SQL(
                    """UPDATE {table}
                    SET {column} = {expression}
                    RETURNING *
                """)
                .format(
                    table=sql.Identifier("products"),
                    column=sql.Identifier("price"),
                    expression=expression
                )
            ) 
        # check for errors
        except psycopg.ProgrammingError as e: 
            raise psycopg.ProgrammingError(f"""
Problem running query:
    {e}

Are you sure every name is spelled correctly?
You can use show all tables or columns to check db contents.
            """)

        # print the results
        while True:
            for row in cur.fetchall():
                print(row)
            if not cur.nextset():
                break

Conclusions

Updating tables might be simple or very complicated depending on your use case. With this recipe those simple cases will become almost instant.

Recipes used in the postgresql-python-update-column.ipynb

All code recipes used in this notebook are listed below. You can click them to check their documentation.

Packages used in the postgresql-python-update-column.ipynb

List of packages that need to be installed in your Python environment to run this notebook. Please note that MLJAR Studio automatically installs and imports required modules for you.

psycopg>=3.2.1

python-dotenv>=1.0.1

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