The DDL is comprised of the Create, Read, Update, Delete (CRUD) operations which form the backbone of any SQL database system. Let us discuss how to perform CRUD operations on a PostgreSQL database using python. Pyscopg2 is the most preferred mode and is widely used to connect the PostgreSQL database using python. Pyscopg2 is a Database API which is a PostgreSQL compatible driver that manages connection pools.
In this article, we will learn how to connect to the PostgreSQL database using SQLAlchemy in python and also understood the procedure on how to perform CRUD operations against the PostgreSQL database.
Establishing a connection with the PostgreSQL database
As the first steps establish a connection with your existing database, using the connect() function of Psycopg2.
Python3
from psycopg2 import connect
# declare the connection string specifying# the host name database name# use name and passwordconn_string = "host='host_name' \
dbname='database_name' user='user_name'\
password='your_password'"
# use connect function to establish the connectionconn = connect(conn_string)
|
Explanation:
host – name of the host in which the database is hosted
Username – Name of the admin
Password – Password of the admin
dbname– database name
Performing Create operation in PostgreSQL database using Python
- The syntax for creating a table is similar to that of the syntax used in conventionally SQL statements. Create different tables with required columns and column constraints as shown. Establish a connection with the PostgreSQL using connect() as discussed above
- Now, instantiate a new cursor() object. A cursor is a read-only pointer that allows a program, to access the result set of a query.
- Drop any tables in the same name if already exists. Then, pass the query for creating a table to the execute() function of psycopg2.
- Finally call the commit() method, indicating the connector object which was created to establish a connection with the database to commit all changes to the database. This will ensure that the tables are created.
Python3
import psycopg2
# Establishing the connectionconn = psycopg2.connect(
database="databasename",
user='username',
password='password',
host='hostname',
port='5432'
)# Creating a cursor object using the # cursor() methodcursor = conn.cursor()
# Doping EMPLOYEE table if already exists.cursor.execute("DROP TABLE IF EXISTS publisher")
# Creating table as per requirementsql = '''CREATE TABLE PUBLISHER(
publisher_id SERIAL PRIMARY KEY,
publisher_name VARCHAR(255) NOT NULL,
publisher_estd INT,
publsiher_location VARCHAR(255),
publsiher_type VARCHAR(255)
)'''cursor.execute(sql)print("Table created successfully........")
conn.commit()# Closing the connectionconn.close() |
Output:
Table created successfully........
Creating a table in PostgreSQL using Python
Performing Insert operation in PostgreSQL database using Python
- The syntax for inserting records in a table is similar to that of the syntax used in conventionally SQL statements. Establish a connection with the PostgreSQL database as discussed above.
- Now, instantiate a new cursor() object. A cursor is a read-only pointer that allows a program, to access the result set of a query.
- Then, pass the query for inserting table records to the execute() function of psycopg2.
- Finally call the commit() method, indicating the connector object which was created to establish a connection with the database to commit all changes to the database. This will ensure that the records are inserted.
Python3
import psycopg2
try:
connection = psycopg2.connect(user="username",
password="password",
host="127.0.0.1",
port="5432",
database="databasename")
cursor = connection.cursor()
postgres_insert_query = """ INSERT INTO publisher(publisher_id,
publisher_name, publisher_estd, publsiher_location, publsiher_type)
VALUES (%s,%s,%s,%s,%s)"""
record_to_insert = [(1, 'Packt', 1950,
'chennai', 'books'),
(2, 'Springer', 1950,
'chennai', 'books'),
(3, 'Springer', 1950,
'chennai', 'articles'),
(4, 'Oxford', 1950,
'chennai', 'all'),
(5, 'MIT', 1950,
'chennai', 'books')]
for i in record_to_insert:
cursor.execute(postgres_insert_query, i)
connection.commit()
count = cursor.rowcount
print(count, "Record inserted successfully \
into publisher table")
except (Exception, psycopg2.Error) as error:
print("Failed to insert record into publisher table", error)
finally:
# closing database connection.
if connection:
cursor.close()
connection.close()
print("PostgreSQL connection is closed")
|
Output:
1 Record inserted successfully into publisher table
PostgreSQL connection is closed
Inserting records in a table in PostgreSQL using Python
Performing Read operation in PostgreSQL database using Python
- Establish a connection with the PostgreSQL database as discussed above.
- Now, instantiate a new cursor() object. A cursor is a read-only pointer that allows a program, to access the result set of a query.
- Then, pass the query for creating a table to the execute() function of psycopg2.
- The query result will be stored in the cursor object named the engine. Use fetchall() method to get all the rows of the resulting query.
- Now, iterate through each row to see the query result as shown. In order to capture any errors while selecting the records in a database and close the connection smoothly after committing all changes, use try, expect, and finally block as shown in the below code.
Python3
import psycopg2
try:
connection = psycopg2.connect(user="username",
password="password",
host="hostname",
port="5432",
database="databasename")
cursor = connection.cursor()
postgreSQL_select_Query = "select * from publisher"
cursor.execute(postgreSQL_select_Query)
print("Selecting rows from publisher table using cursor.fetchall")
publisher_records = cursor.fetchall()
print("Print each row and it's columns values")
for row in publisher_records:
print("publisher_Id = ", row[0], )
print("publisher_name = ", row[1])
print("publisher_estd = ", row[2])
print("publisher_location = ", row[3])
print("publisher_type = ", row[4], "\n")
except (Exception, psycopg2.Error) as error:
print("Error while fetching data from PostgreSQL", error)
finally:
# closing database connection.
if connection:
cursor.close()
connection.close()
print("PostgreSQL connection is closed")
|
Output:
Reading records in a table in PostgreSQL using Python
Performing Update operation in PostgreSQL database using Python
The syntax for updating a table is similar to that of the syntax used in conventionally SQL statements. Here we write an update query that updates publisher name by publisher id as shown.
- Establish a connection with the PostgreSQL database as discussed above.
- Now, instantiate a new cursor() object. A cursor is a read-only pointer that allows a program, to access the result set of a query.
- Then, pass the query for updating a table to the execute() function of psycopg2.
- Use rowcount function to count the number of rows updated.
- Finally call the commit() method, indicating the connector object which was created to establish a connection with the database to commit all changes to the database. This will ensure that the tables are updated.
- In order to capture any errors while updating tables in a database and close the connection smoothly after committing all changes, use try, expect, and finally block as shown in the below code.
Test the update_publisher() function by calling the function and checking for the updates.
Python3
import psycopg2
def updateTable(publisherId, establishedYear):
try:
connection = psycopg2.connect(user="username",
password="password",
host="hostname",
port="5432",
database="databasename")
cursor = connection.cursor()
# Update single record now
sql_update_query = """Update publisher set \
publisher_estd = %s where publisher_id = %s"""
cursor.execute(sql_update_query,
(establishedYear,
publisherId))
connection.commit()
count = cursor.rowcount
print(count, "Record Updated successfully ")
except (Exception, psycopg2.Error) as error:
print("Error in update operation", error)
finally:
# closing database connection.
if connection:
cursor.close()
connection.close()
print("PostgreSQL connection is closed")
# call the update functionpublisherId = 3
establishedYear = 2000
updateTable(publisherId, establishedYear) |
Output:
1 Record Updated successfully PostgreSQL connection is closed
Here, row corresponding to id = 3 is updated with a new value for publsher_estd.
Updating records in a table in PostgreSQL using Python
Performing Delete operation in PostgreSQL database using Python
The syntax for deleting a table is similar to that of the syntax used in conventionally SQL statements. Here we will write a delete query that deletes the record by publisher id as shown.
- Establish a connection with the PostgreSQL database as discussed above.
- Now, instantiate a new cursor() object. A cursor is a read-only pointer that allows a program, to access the result set of a query.
- Then, pass the query for deleting a record in the table to the execute() function of psycopg2.
- Use rowcount function to count the number of rows deleted.
- Finally call the commit() method, indicating the connector object which was created to establish a connection with the database to commit all changes to the database. This will ensure that the records are deleted.
- In order to capture any errors while deleting records in a database and close the connection smoothly after committing all changes, use try, expect, and finally block as shown in the below code.
Test the delete_publisher() function by calling the function and check for the updates.
Python3
import psycopg2
def deleteData(publisherId):
try:
connection = psycopg2.connect(user="username",
password="password",
host="hostname",
port="5432",
database="databasename")
cursor = connection.cursor()
# Update single record now
sql_delete_query = """Delete from publisher\
where publisher_id = %s"""
cursor.execute(sql_delete_query, (publisherId,))
connection.commit()
count = cursor.rowcount
print(count, "Record deleted successfully ")
except (Exception, psycopg2.Error) as error:
print("Error in Delete operation", error)
finally:
# closing database connection.
if connection:
cursor.close()
connection.close()
print("PostgreSQL connection is closed")
publisherId = 4
deleteData(publisherId) |
Output:
1 Record deleted successfully PostgreSQL connection is closed
Here, the row with id = 4 has been deleted
Deleting records in a table in PostgreSQL using Python