How do you test for the existence of a user in SQL Server?

Question

I'd like to drop a user in a SQL Server script but I'll need to test for existence first or I'll get script errors. When dropping tables or stored procs, I check the sysobjects table like so:

IF EXISTS (SELECT * FROM sysobjects WHERE id = object_id(N'[dbo].[up_SetMedOptions]') AND OBJECTPROPERTY(id, N'IsProcedure') = 1) Drop Procedure up_SetMedOptions; GO

What is the corollary for checking for a user? Note that I am NOT asking about a database login to the server! The question pertains to a User in a specific database.

Answer 1

In SQL 2005:

select * from sys.sysusers

In SQL 2000:

select * from sysusers

Answer 2

SSMS scripts it in the following way:

For SQL 2005/2008:

IF  EXISTS (SELECT * FROM sys.database_principals WHERE name = N'username')
DROP USER [username]

For SQL 2000:

IF  EXISTS (SELECT * FROM dbo.sysusers WHERE name = N'username')
EXEC dbo.sp_revokedbaccess N'username'