Multi-row UPSERT (INSERT or UPDATE) from Python

Question

I am currently executing the simply query below with python using pyodbc to insert data in SQL server table:

import pyodbc

table_name = 'my_table'
insert_values = [(1,2,3),(2,2,4),(3,4,5)]

cnxn = pyodbc.connect(...)
cursor = cnxn.cursor()
cursor.execute(
    ' '.join([
        'insert into',
        table_name,
        'values',
        ','.join(
            [str(i) for i in insert_values]
        )
    ])
)
cursor.commit()

This should work as long as there are no duplicate keys (let's assume the first column contains the key). However for data with duplicate keys (data already existing in the table) it will raise an error. How can I, in one go, insert multiple rows in a SQL server table using pyodbc such that data with duplicate keys simply gets updated.

Note: There are solutions proposed for single rows of data, however, I would like to insert multiple rows at once (avoid loops)!

Answer 1

This can be done using MERGE. Let's say you have a key column ID, and two columns col_a and col_b (you need to specify column names in update statements), then the statement would look like this:

MERGE INTO MyTable as Target
USING (SELECT * FROM 
       (VALUES (1, 2, 3), (2, 2, 4), (3, 4, 5)) 
       AS s (ID, col_a, col_b)
      ) AS Source
ON Target.ID=Source.ID
WHEN NOT MATCHED THEN
INSERT (ID, col_a, col_b) VALUES (Source.ID, Source.col_a, Source.col_b)
WHEN MATCHED THEN
UPDATE SET col_a=Source.col_a, col_b=Source.col_b;

You can give it a try on rextester.com/IONFW62765.

Basically, I'm creating a Source table "on-the-fly" using the list of values, which you want to upsert. When you then merge the Source table with the Target, you can test the MATCHED condition (Target.ID=Source.ID) on each row (whereas you would be limited to a single row when just using a simple IF <exists> INSERT (...) ELSE UPDATE (...) condition).

In python with pyodbc, it should probably look like this:

import pyodbc

insert_values = [(1, 2, 3), (2, 2, 4), (3, 4, 5)]
table_name = 'my_table'
key_col = 'ID'
col_a = 'col_a'
col_b = 'col_b'

cnxn = pyodbc.connect(...)
cursor = cnxn.cursor()
cursor.execute(('MERGE INTO {table_name} as Target '
                'USING (SELECT * FROM '
                '(VALUES {vals}) '
                'AS s ({k}, {a}, {b}) '
                ') AS Source '
                'ON Target.ID=Source.ID '
                'WHEN NOT MATCHED THEN '
                'INSERT ({k}, {a}, {b}) VALUES (Source.{k}, Source.{a}, Source.{b}) '
                'WHEN MATCHED THEN '
                'UPDATE SET {k}=Source.{a}, col_b=Source.{b};'
                .format(table_name=table_name,
                        vals=','.join([str(i) for i in insert_values]),
                        k=key_col,
                        a=col_a,
                        b=col_b)))
cursor.commit()

You can read up more on MERGE in the SQL Server docs.