INSERT INTO if not exists SQL server

Question

I have a database structured as follows:

users

userid (Primary Key)
username

group

Answer 1

@gbn answer needs SQL server 2008 or higher. I tried a non-merge way to do it. Ugly perhaps, but it works.

declare @user varchar(50)
set @user = 'username'
insert into users (username) 
select @user
where not exists (
 select username 
 from users 
 where username = @user
);

If you want to test any of the answers, here is the SQL for the table -

CREATE TABLE [dbo].[users](
    [userid] [int] NULL,
    [username] [varchar](50) NULL
)

INSERT [dbo].[users] ([userid], [username]) VALUES (1, N'John')
INSERT [dbo].[users] ([userid], [username]) VALUES (2, N'David')
INSERT [dbo].[users] ([userid], [username]) VALUES (3, N'Stacy')
INSERT [dbo].[users] ([userid], [username]) VALUES (4, N'Arnold')
INSERT [dbo].[users] ([userid], [username]) VALUES (5, N'Karen')