sql存储过程无重复添加修改

CREATE PROCEDURE [dbo].[ProcAddTeacherInfo](
@UN varchar(50),
@TN nvarchar(50),
@TT varchar(50),
@TE varchar(50),
@TG bit,
@szm varchar(50)
)
AS
BEGIN
if not exists (select * from Teacher where UserNum=@UN)
begin
insert into Teacher(UserNum,TeacherName,TeacherTel,TeacherEmail,TeacherGender,szm)values(@UN,@TN,@TT,@TE,@TG,@szm)
end
else 
begin
update Teacher set TeacherName=@TN,TeacherTel=@TT,TeacherEmail=@TE,TeacherGender=@TG,szm=@szm where UserNum=@UN
end
END

查询是否存在相同的usernum 若有 则修改信息，若无 则添加