not in tab1 or not in tab2

[sql]

My table 1 and tab2 both have primary key call student_id , if I want to do if the id is not in tab1 or not in tab2 then do insert ???

[Civic1986]

if exists(select tab1.student_id,tab2.student_id from tab1,tab2 where tab1.student_id=@student_id or tab2.student_id=@student_id )
begin
////code when a student_id exists
end
else
//// Insert query
begin
end