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

SSMS scripts it in the following way:

For SQL 2005/2008 and later:

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'

Answer 2

In SQL 2005:

select * from sys.sysusers

In SQL 2000:

select * from sysusers

Answer 3

The code below worked for me.

IF EXISTS (SELECT * FROM sys.syslogins WHERE name = N'MyUserName') 
DROP LOGIN [MyUserName]