In today’s world, mergers and acquisitions of organizations are a reality. Having been through a couple of M&As in the last couple of years myself, I have had a unique opportunity to experience changes which would not be experienced in the normal course of work.

One such change is the changing of the domain logins. M&As often come with renaming of domain logins to confirm to a common standard. The question that came up in our team was:

What happens to our SQL Servers that use windows authentication if the login name changes?

The answer? Nothing. SQL Server continues to work as usual if the login name is the only thing that changed (and your applications do not use the physical “login name” in any way).

Here’s a demo for clarity.

On my VM, I created a user “OldUser” and logged in to SSMS using Windows Authentication when running as that user.

Using an Old Login to login to SSMS

For the record, we will also execute the following query:

SELECT SUSER_SNAME() AS LoginName,SUSER_SID() AS LoginSID;

OldUser’s SID

Now, because this is a local login, all I do is rename the Windows login using Computer Management. The same behaviour applies to logins renamed via Active Directory as well.

Renaming the “OldUser” to “NewUser”

I load the SSMS again – this time as “NewUser”.

Login to SSMS using the new, renamed login

Notice how the authentication works even though the login name has changed.

Wrapping it up…

Windows authentication in SQL Server is based on the SID. Hence, even though the login name changed, SQL Server allowed me to login because the SID did not change. In fact, this is why when we create or modify a login, the only requirement is to ensure that the SID of the login is same as the one we want to create/modify.

In a practical scenario, the IT teams would not re-generate the SIDs because that would mean too much work – realigning file & folder shares, resetting SSO and what have you. Instead, they simply update the login name (or the human interpretative part of the login).

What this means is that the authentication on the SQL Servers continue to work as usual – even after the login changes. No manual intervention is required!

Note of caution: Because the SQL Login name and the Windows login name are independent, renaming the Windows login will NOT rename the SQL login. The entry under sys.server_principals continues to read the old value. So, if your applications are designed to work based on the login name in sys.server_principals, you need to use ALTER LOGIN statements whenever IT changes the Windows login names.

Further Reading

• CREATE LOGIN [MSDN Link]
• ALTER LOGIN [MSDN Link]

In case you are really interested in digging deeper into SQL Server,

• Security considerations for a SQL Server installation [MSDN Link]

Until we meet next time,
Be courteous. Drive responsibly.