SUSER_NAME (Transact-SQL)
Returns the login identification name of the user.
In SQL Server version 7.0, the security identification number (SID) replaced the server user identification number (SUID).
SUSER_NAME returns a login name only for a login that has an entry in the syslogins system table.
SUSER_NAME can be used in a select list, in a WHERE clause, and anywhere an expression is allowed, and must always be followed by parentheses, even if no parameter is specified.

