What's wrong in this mysql query?

Question

I was trying to insert rows into a table called subjects , i wrote the following query so that the duplicates can't occur. But this is not working.

INSERT INTO subjects ( stu_id, sub_id ) VALUES (16, 39) 
WHERE NOT EXISTS ( SELECT * FROM subjects WHERE stu_id = 16 AND sub_id = 39 );

Answer 1

INSERT IGNORE INTO subjects ( stu_id, sub_id ) VALUES (16, 39) 

This will ignore the data, if there are allready rows with that kind of data-combnation in a unique key.