How to build a generic function in Python to Insert and update records?

Question

I wanted to build a generic function in Python to Insert a record into table A from table B if the record does not exists in table A else update with the data if the primary key is identified. I am not able to get the result with the below code:

def insert_update_record(table_A, table_B):
    insert_query = "INSERT INTO {} SELECT * FROM {} WHERE NOT EXISTS (SELECT 1 FROM {} WHERE {}.id = {}.id)".format(table_A, table_B, table_A, table_B, table_A)
    update_query = "UPDATE {} SET {} = {} FROM {} WHERE {}.id = {}.id".format(table_A, table_A, table_B, table_A, table_B, table_A)
    
    cur.execute(insert_query)
    cur.execute(update_query)
    conn.commit()

Asked By: yocodefreak

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Source

Answer 1

Python uses c.execute(q) function to update a record from a table where c is cursor and q is the update query to be executed.

Syntax:

# execute SQL query using execute() method.
cursor.execute(sql)
cursor.commit()
# get the record count updated
print(mycursor.rowcount, "record (s) affected")

Answered By: Pranav Gupta

Answer 2

First, I should say SELECT 1 in your insert_query is not ok and you probably need to change it to something like this:

insert_query = "INSERT INTO {} SELECT * FROM {} WHERE NOT EXISTS (SELECT * FROM {} WHERE {}.id = {}.id) LIMIT 1".format(table_A, table_B, table_A, table_B, table_A)

But what you want to implement is a concept called Upsert in database concepts and I do it for you using INSERT ON CONFLICT statement like below:

import psycopg2


def connect():
    try:
        conn = psycopg2.connect(database ="name_of_your_database",
                            user = "user_of_this_database",
                            password = "password_forthis_user",
                            host = "localhost",
                            port = "5432")

        cur = conn.cursor()
    
    except (Exception, psycopg2.DatabaseError) as error:
        print ("Error while creating PostgreSQL table", error)
    
    return conn, cur

def insert_update_record(table_A, table_B):
    conn, cur = connect()
    upsert_query="""
INSERT INTO {}
SELECT * FROM {} LIMIT 1
ON CONFLICT (id) 
DO 
   UPDATE SET field_name = EXCLUDED.field_name || ';' || {}.field_name;
""".format(table_A, table_B, table_B)

    cur.execute(upsert_query)
    conn.commit()

if __name__ == '__main__':
    insert_update_record("table_name_1", "table_name_2")

Reference:

To learn more about this concept in Postgres or other databases you can tour the below links:

Psycopg2: how to insert and update on conflict using psycopg2 with python? ^{[stackoverflow]}

Insert, on duplicate update in PostgreSQL? ^{[stackoverflow]}

PostgreSQL Upsert Using INSERT ON CONFLICT statement ^{[POSTGRESQL TUTORIAL]}

How to UPSERT (MERGE, INSERT … ON DUPLICATE UPDATE) in PostgreSQL? ^{[stackoverflow]}

Idiomatic way to implement UPSERT in PostgreSQL ^{[dba-stackexchange]}

Answered By: Javad