Can't execute query

 SELECT c.id as cid, c.name as category
 FROM categories c
 WHERE c.parent_id = 
 ORDER by c.name, c.id
 

MySQL Error: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'ORDER by c.name, c.id' at line 4

This script cannot continue, terminating.