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Python Select from PostgreSQL Table.Filed Under: Python PostgreSQL | December 17, 2019 2 Comments.In this article, we will learn how to execute a PostgreSQL SELECT query from a Python application to fetch data from the database table using Psycopg2 . Also, learn how to process and use result set returned by SELECT Query.Goals of this lesson. In this lesson, you’ll learn the following PostgreSQL SELECT operations from Python:Retrieve all rows from the PostgreSQL table using fetchall(), and limited rows using fetchmany()and fetchone(). Use Python variables in the where clause of a PostgreSQL SELECT query to pass dynamic values.The simple exercise to quickly learn Python database programming skills.The quiz provides MCQ to get familiar with the Python Database API Specification.Further Reading : A Detailed Tutorial on Python PostgreSQL Tutorial (6 Article)Before we start.Before executing the following programs, please make sure you have the following in place: –Username and password that you need to connect PostgreSQL PostgreSQL database table from which you want to fetch data.For this article, I am using a mobile table created in my PostgreSQL database. If no table is present in your PostgreSQL server you can refer to our article to create a PostgreSQL table from Python.Steps to perform a PostgreSQL SELECT query from Python.Install psycopg2 using pip. Create a PostgreSQL database connection . Define the SELECT statement query to fetch data from the PostgreSQL table. Execute the SELECT query using a cursor.execute() and get a python ResultSet. Iterate over the ResultSet using for loop to get the database fields (columns) from each row. Close the cursor and database connection. Catch any SQL exceptions that may occur during the process.Let see the example now.Python example to retrieve data from PostgreSQL Table using fetchall()In this example, Let see how to fetch all rows from the database table.Note : In the above example, we used cursor.fetchall() to get all the rows of a database table.Use cursor.execute() to run a query then use.cursor. fetchall () to fetch all rows. cursor. fetchone () to fetch single row. cursor. fetchmany (SIZE) to fetch limited rows.Pass Python variable as parameters in PostgreSQL Select Query.Most of the time we need to pass python variables as parameters to SQL queries to get the result. For example, the application can pass any user id to get the user details to handle such requirements we need to use a parameterized query.A parameterized query is a query in which we use placeholders (%s) for parameters , and the parameter values supplied at execution time. That means parameterized query compiled only once.Output :Retrieve a limited number of rows from PostgreSQL table.In most of the situation retrieving all of the rows from a table can be time-consuming if the table contains thousands of rows.So a better alternative is to retrieve a few rows using a cursor.fetchmany() .Syntax of fetchmany() .Here size is the number of rows to be retrieved . Thi s method fetches the next set of rows from a query result . fetchmany() method return a list of tuple contains the rows. fetchmany returns an empty list when no more rows are available in the table. The number of rows to fetch depends on the SIZE argument. A ProgrammingError raised if the previous call to execute*() d >Python example to fetch limited rows from PostgreSQL table using a cursor.fetchmany()Using cursor.fetchone.Use a cursor.fetchone() to retrieve an only single row from PostgreSQL table in Python. You can also use cursor.fetchone() to fetch the next row of a query result set. This method returns a single tuple . It can return a none if no rows are available in the resultset. The cursor.fetchall() and fetchmany() method internally uses this method.Python example to retrieve a single row from PostgreSQL table using cursor.fetchone .Next Steps:To practice what you learned in this article, I have created a Python Database programming Quiz and Exercise Project.Solve our Python PostgreSQL Database programming Quiz to test your Python database concepts. Solve our Python PostgreSQL Database Exercise project to Practice and master the Python Database operations.That’s it. Folks Let me know your comments and questions in the section below.

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