Select every column from table using python

Selecting every row and column from a table is the simplest way to check table contents. This recipe is best used for those type of scenarios.

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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()

Let's list all tables in out database, you can do so using show all tables recipe. We want to focus on test table.

# 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 table_name
                FROM information_schema.tables
                WHERE table_type = 'BASE TABLE'
                AND table_schema NOT IN ('pg_catalog', 'information_schema');
            """)
        # 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("Tables:")
        for table in cur.fetchall():
            print(f"{table}")

Lets select every column from this table using *. You can also list a custom list of columns by simply listing them in comma separated list.

# 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("test"),
                )
            )
        # 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}")

Conclusions

Selecting contents of a table if common and very useful debuging toll while working whit with any type of database. This recipe is very useful.

Recipes used in the postgresql-python-select-query.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-select-query.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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