Checking if a SQL Server login already exists

Here’s a way to do this in SQL Server 2005 and later without using the deprecated syslogins view:

IF NOT EXISTS 
    (SELECT name  
     FROM master.sys.server_principals
     WHERE name="LoginName")
BEGIN
    CREATE LOGIN [LoginName] WITH PASSWORD = N'password'
END

The server_principals view is used instead of sql_logins because the latter doesn’t list Windows logins.

If you need to check for the existence of a user in a particular database before creating them, then you can do this:

USE your_db_name

IF NOT EXISTS
    (SELECT name
     FROM sys.database_principals
     WHERE name="Bob")
BEGIN
    CREATE USER [Bob] FOR LOGIN [Bob] 
END