SELECT 
  a.category_id 
FROM 
  cscart_categories as a 
  LEFT JOIN cscart_categories as b ON b.category_id IN (261) 
WHERE 
  a.id_path LIKE CONCAT(b.id_path, '/%')

Query time 0.00045

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE b const PRIMARY,p_category_id,idx_category_id PRIMARY 3 const 1
1 SIMPLE a ALL 415 Using where

Result

category_id
365
359
901
369
263
948
949
950
480
903
923
926
928
951
952
953
954
955
956
957
958
959
960
986
987
988
989
990
991
992
993
994
995
996
997