SQL SERVER: crear usuario y login si no existen

IF NOT EXISTS 

    (SELECT name  

     FROM master.sys.server_principals

     WHERE name = 'ff')

BEGIN

    CREATE LOGIN ff WITH PASSWORD = N'ff', CHECK_POLICY = OFF

END

GO

IF NOT EXISTS 

    (SELECT 1   

     FROM DBO.SYSUSERS

     WHERE name = 'ff')

BEGIN

CREATE USER ff FROM LOGIN ff

EXEC SP_ADDROLEMEMBER 'db_datawriter', 'ff'

EXEC SP_ADDROLEMEMBER 'db_datareader', 'ff'

END

GO