Python PostgreSQL - Join

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When you have divided the data in two tables you can fetch combined records from these two tables using Joins.

Example

Assume we have created a table with name CRICKETERS and inserted 5 records into it as shown below −

postgres=# CREATE TABLE CRICKETERS (
   First_Name VARCHAR(255), Last_Name VARCHAR(255), Age int, 
   Place_Of_Birth VARCHAR(255), Country VARCHAR(255)
);
postgres=# insert into CRICKETERS values (
   'Shikhar', 'Dhawan', 33, 'Delhi', 'India'
);
postgres=# insert into CRICKETERS values (
   'Jonathan', 'Trott', 38, 'CapeTown', 'SouthAfrica'
);
postgres=# insert into CRICKETERS values (
   'Kumara', 'Sangakkara', 41, 'Matale', 'Srilanka'
);
postgres=# insert into CRICKETERS values (
   'Virat', 'Kohli', 30, 'Delhi', 'India'
);
postgres=# insert into CRICKETERS values (
   'Rohit', 'Sharma', 32, 'Nagpur', 'India'
);

And, if we have created another table with name OdiStats and inserted 5 records into it as −

postgres=# CREATE TABLE ODIStats (
   First_Name VARCHAR(255), Matches INT, Runs INT, AVG FLOAT, 
   Centuries INT, HalfCenturies INT
);
postgres=# insert into OdiStats values ('Shikhar', 133, 5518, 44.5, 17, 27);
postgres=# insert into OdiStats values ('Jonathan', 68, 2819, 51.25, 4, 22);
postgres=# insert into OdiStats values ('Kumara', 404, 14234, 41.99, 25, 93);
postgres=# insert into OdiStats values ('Virat', 239, 11520, 60.31, 43, 54);
postgres=# insert into OdiStats values ('Rohit', 218, 8686, 48.53, 24, 42);

Following statement retrieves data combining the values in these two tables −

postgres=# SELECT
Cricketers.First_Name, Cricketers.Last_Name, Cricketers.Country,
OdiStats.matches, OdiStats.runs, OdiStats.centuries, OdiStats.halfcenturies
from Cricketers INNER JOIN OdiStats ON Cricketers.First_Name = OdiStats.First_Name;
first_name  | last_name  | country     | matches | runs  | centuries | halfcenturies
------------+------------+-------------+---------+-------+-----------+---------------
Shikhar     | Dhawan     | India       | 133     | 5518  | 17        | 27
Jonathan    | Trott      | SouthAfrica | 68      | 2819  | 4         | 22
Kumara      | Sangakkara | Srilanka    | 404     | 14234 | 25        | 93
Virat       | Kohli      | India       | 239     | 11520 | 43        | 54
Rohit       | Sharma     | India       | 218     | 8686  | 24        | 42
(5 rows)
postgres=#

Joins using python

When you have divided the data in two tables you can fetch combined records from these two tables using Joins.

Example

Following python program demonstrates the usage of the JOIN clause −

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 single row
sql = '''SELECT * from EMP INNER JOIN CONTACT ON EMP.CONTACT = CONTACT.ID'''

#Executing the query
cursor.execute(sql)

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

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

#Closing the connection
conn.close()

Output

[('Ramya', 'Rama priya', 27, 'F', 9000.0, 101, 101, 'Krishna@mymail.com', 'Hyderabad'), 
   ('Vinay', 'Battacharya', 20, 'M', 6000.0, 102, 102, 'Raja@mymail.com', 'Vishakhapatnam'), 
   ('Sharukh', 'Sheik', 25, 'M', 8300.0, 103, 103, 'Krishna@mymail.com ', 'Pune'), 
   ('Sarmista', 'Sharma', 26, 'F', 10000.0, 104, 104, 'Raja@mymail.com', 'Mumbai')]