Bug or inconsistency in SUSER_SID()

Question

SUSER_SID() returns null if the login passed to it contains square brackets.
I experienced this problem when I created a SQL Server credential with the user name containing the square brackets surrounding the user name in the identity.
The credential was created without error.
When I went to create a proxy with sp_add_proxy using the credential I had created, I got a msg 14529 error saying that the user account was not a valid Windows user.
I worked through the source of sp_add_proxy until I found that SUSER_SID() was returning null.
This was tested with SQL Server 2012 and I expect that this error/inconsistency has been there all along. I've certainly found no reference to this anywhere on the internet.
Hopefully this will save someone else a couple of wasted hours!

Answer 1

In a sense you're right; SUSER_SID() probably doesn't have a problem. The inconsistency starts with create credential accepting the identity with enclosing square brackets.
Yes, the square brackets are taken literaly. What you enter is an external identity, and how would SQL Server know that the brackets cannot be part of the external identity?
The purpose of square brackets or double quotes is to delimit identifiers that includes characters that are not permitted in identifiers, for instance backslash. But since the identity is entered as a string literal, there is no need for quoting in this
case.
Erland Sommarskog, SQL Server MVP, [email protected]