How can I perform an UPDATE if EXISTS based on existing data in SQL Server

Question

I am pulling data from an application (via Python) and writing that data into a SQL Server database. My initial data loads, however I will need to UPDATE that data based on subsequent API retrievals from my Python script. While I am successfully able to update all existing rows in the my database I only want to update the values that are different. To visualize:

dev_name    userID
========    ======
abc          def
ghi          jkl
mno          NULL

When Python pulls new data and sees a different uid value for row ‘mno’ for example, the UPDATE statement should only update that row. Non NULL columns could be updated as well. I thought the following should have worked, however it doesn’t.

rslt = cursor.execute(IF EXISTS(SELECT device_name from Devices where device_name = ?) UPDATE Devices SET userID = (?) where device_name = ? and userID <> ?", dev, uid, dev, uid)

To clarify, the parameters of dev and uid refer to the Python variables that contain the data from the API call.

Any clarification or explanation is greatly appreciated.

Asked By: Chad

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Source

Answer 1

In SQL, the WHERE predicate is how you determine existence of a record.

Presumably you want to set the userID to the current owner of the device. Supposing the new userID is ‘pqr’, then:

UPDATE Devices 
SET userID = 'pqr'
where device_name = 'mno'
and userID <> 'pqr';

So for this specific example:

query = """UPDATE Devices 
           SET userID = ?
           where device_name = ?
           and userID <> ? """
args = ('pqr', 'mno', 'pqr')
cursor.execute(query, args)

Or in your case:

query = """UPDATE Devices 
           SET userID = ?
           where device_name = ?
           and userID <> ? """
args = (uid, dev, uid)
cursor.execute(query, args)

Reference

EDIT to handle NULL values:

query = """UPDATE Devices 
           SET userID = ? 
           WHERE device_name = ? 
           AND COALESCE(userID,'NULL') <> COALESCE(?,'NULL'))) """ 
args = (uid,dev,uid)

Answered By: FlexYourData

How can I perform an UPDATE if EXISTS based on existing data in SQL Server

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