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SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(
    705, 706, 707, 708, 709, 710, 711, 712, 
    713, 714, 715, 716, 717, 718, 719, 720
  ) 
  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		16	Using index condition; Using where