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SQL Server silently renames your user when you ALTER with a login

With all this AI features available, you would expect that you no longer loose time on stupid issues. Unfortunately, we are not there yet. I lost a chunk of time today to a behavior in SQL Server that, once you know it, is totally obvious — but until then is absolutely maddening. I'm sharing it here so hopefully you don't lose the same time I did.

The setup

I had a script meant to be idempotent: create a database user if it doesn't exist, or update it if it does. Standard stuff. Here's a simplified version:

Looks fine, right? Run it once — works. Run it a second time and SQL Server throws an error saying it can't find usr_SampleDB_reader. The user you just created. In the same database. With the same script.

What's actually happening

When you run ALTER USER [...] WITH LOGIN = [...], SQL Server renames the user to match the login name — by default, silently, without a warning.

So after the first run, usr_SampleDB_reader no longer exists. It's been renamed to lg_SampleDB_dev_db-reader to match the login. The second run's IF EXISTS check looks for usr_SampleDB_reader, finds nothing, and falls into the ELSE branch — where the CREATE USER fails because the login is already mapped to a user (just under a different name now).

It's one of those behaviors that makes sense once you understand the data model, but gives you zero indication it's happening while it's happening.

The fix

Explicitly tell SQL Server to keep the username as-is by adding NAME = to the ALTER USER statement:

Adding WITH NAME = [usr_IVO_reader] tells SQL Server: yes, remap the login, but don't touch the username. Now the script is actually idempotent.

Why does SQL Server do this?

SQL Server maintains a distinction between logins (server-level principals) and users (database-level principals). When you link a user to a login via ALTER USER ... WITH LOGIN without specifying a name, SQL Server assumes you want them in sync — so it renames the user to match the login. It's following a convention, just not one you'd expect if you're used to treating usernames as stable identifiers.

Once you know this, the fix is obvious. Getting there the first time, though? Less so.

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