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SELECT 
  a.category_id 
FROM 
  cscart_categories as a 
  LEFT JOIN cscart_categories as b ON b.category_id IN (734) 
WHERE 
  a.id_path LIKE CONCAT(b.id_path, '/%')

Query time 0.00031

Explain
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	b	const	PRIMARY,p_category_id	PRIMARY	3	const	1
1	SIMPLE	a	ALL					214	Using where