Change properties of a credential.
Syntax ALTER CREDENTIAL credential WITH IDENTITY = 'identity' [, SECRET = 'secret']
A credential is a record of the authentication information required to connect to a resource outside SQL Server. Most credentials are a Windows username/password.
When IDENTITY is a Windows username, the SECRET can be the password. Secret is encrypted using the service master key.
ALTER CREDENTIAL MyCredential WITH IDENTITY = 'JDoe';
The above will set the secret (password) for MyCredential to NULL
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