SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(734, 557, 603) 
  AND category_id != 264

Query time 0.00026

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE cscart_categories range PRIMARY,parent,p_category_id parent 3 26 Using index condition; Using where

Result

category_id parent_id
734 557
735 557
736 557
737 557
738 557
739 557
740 557
741 557
742 557
743 557
744 557
745 557
746 557
747 557
748 557
555 603
556 603
557 603
604 603
558 603
559 603
560 603
561 603
562 603
563 603
564 603