这个存储过程怎么写?

我想在用户表里创建一个新用户,如果给出参数用户名已经存在返回
一个false值,否则insert一个新用户!请问该怎么写?谢谢!
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if not exists (select * from usertable where username = @name)
insert into usertable (name) values (@name)
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CREATE PROCEDURE up_adduser (@name varchar(10),@rtn bit output)
AS
if not exists (select * from usertable where username = @name)
insert into usertable (name) values (@name)
else
return 0