Python PostgreSQL - Select Data

You can retrieve the contents of an existing table in PostgreSQL using the SELECT statement. At this statement, you need to specify the name of the table and, it returns its contents in tabular format which is known as result set.
Following is the syntax of the SELECT statement in PostgreSQL −
SELECT column1, column2, columnN FROM table_name;

Assume we have created a table with name CRICKETERS using the following query −
postgres=# CREATE TABLE CRICKETERS ( 
   First_Name VARCHAR(255), Last_Name VARCHAR(255), Age int, 
   Place_Of_Birth VARCHAR(255), Country VARCHAR(255)
);

CREATE TABLE

postgres=#

And if we have inserted 5 records in to it using INSERT statements as −
postgres=# insert into CRICKETERS values('Shikhar', 'Dhawan', 33, 'Delhi', 'India');
INSERT 0 1
postgres=# insert into CRICKETERS values('Jonathan', 'Trott', 38, 'CapeTown', 'SouthAfrica');
INSERT 0 1
postgres=# insert into CRICKETERS values('Kumara', 'Sangakkara', 41, 'Matale', 'Srilanka');
INSERT 0 1
postgres=# insert into CRICKETERS values('Virat', 'Kohli', 30, 'Delhi', 'India');
INSERT 0 1
postgres=# insert into CRICKETERS values('Rohit', 'Sharma', 32, 'Nagpur', 'India');
INSERT 0 1
Following SELECT query retrieves the values of the columns FIRST_NAME, LAST_NAME and, COUNTRY from the CRICKETERS table.
postgres=# SELECT FIRST_NAME, LAST_NAME, COUNTRY FROM CRICKETERS;
 first_name | last_name  | country
------------+------------+-------------
Shikhar     | Dhawan     | India
Jonathan    | Trott      | SouthAfrica
Kumara      | Sangakkara | Srilanka
Virat       | Kohli      | India
Rohit       | Sharma     | India
(5 rows)

If you want to retrieve all the columns of each record you need to replace the names of the columns with "⚹" as shown below −
postgres=# SELECT * FROM CRICKETERS;
first_name  | last_name  | age | place_of_birth | country
------------+------------+-----+----------------+-------------
Shikhar     | Dhawan     | 33  | Delhi          | India
Jonathan    | Trott      | 38  | CapeTown       | SouthAfrica
Kumara      | Sangakkara | 41  | Matale         | Srilanka
Virat       | Kohli      | 30  | Delhi          | India
Rohit       | Sharma     | 32  | Nagpur         | India
(5 rows)

postgres=#

READ Operation on any database means to fetch some useful information from the database. You can fetch data from PostgreSQL using the fetch() method provided by the psycopg2.
The Cursor class provides three methods namely fetchall(), fetchmany() and, fetchone() where,
The fetchall() method retrieves all the rows in the result set of a query and returns them as list of tuples. (If we execute this after retrieving few rows, it returns the remaining ones).
The fetchall() method retrieves all the rows in the result set of a query and returns them as list of tuples. (If we execute this after retrieving few rows, it returns the remaining ones).
The fetchone() method fetches the next row in the result of a query and returns it as a tuple.
The fetchone() method fetches the next row in the result of a query and returns it as a tuple.
Note − A result set is an object that is returned when a cursor object is used to query a table.
The following Python program connects to a database named mydb of PostgreSQL and retrieves all the records from a table named EMPLOYEE.
import psycopg2

#establishing the connection
conn = psycopg2.connect(
   database="mydb", user='postgres', password='password', host='127.0.0.1', port= '5432'
)

#Setting auto commit false
conn.autocommit = True

#Creating a cursor object using the cursor() method
cursor = conn.cursor()

#Retrieving data
cursor.execute('''SELECT * from EMPLOYEE''')

#Fetching 1st row from the table
result = cursor.fetchone();
print(result)

#Fetching 1st row from the table
result = cursor.fetchall();
print(result)

#Commit your changes in the database
conn.commit()

#Closing the connection
conn.close()
('Ramya', 'Rama priya', 27, 'F', 9000.0)
[
   ('Vinay', 'Battacharya', 20, 'M', 6000.0),
   ('Sharukh', 'Sheik', 25, 'M', 8300.0),
   ('Sarmista', 'Sharma', 26, 'F', 10000.0),
   ('Tripthi', 'Mishra', 24, 'F', 6000.0)
]


    187 Lectures 
     17.5 hours 

 Malhar Lathkar

    55 Lectures 
     8 hours 

 Arnab Chakraborty

    136 Lectures 
     11 hours 

 In28Minutes Official

    75 Lectures 
     13 hours 

 Eduonix Learning Solutions

    70 Lectures 
     8.5 hours 

 Lets Kode It

    63 Lectures 
     6 hours 

 Abhilash Nelson
 Print
 Add Notes
 Bookmark this page