SQL table1 row_id不显示table2 row_id

Below is my SQL query

select c.course_id,c.course_title,c.course_credits,c.course_status from       
 course c,(select course_id from completed_course 
 where student_id='1229CSE00241') as a
 where c.course_id!=a.course_id   

But it show all row of a courses also but I want only courses of c which
doesn't in courses of a.

select 
c.course_id,
c.course_title,
c.course_credits,
c.course_status 
from course c 
where 
c.course_id NOT IN (select course_id from completed_course where student_id='1229CSE00241');

select c.course_id,
c.course_title,
c.course_credits,
c.course_status 
from course c where c.course_id not in 
(select course_id from completed_course where student_id='1229CSE00241')