SELECT 
  a.category_id 
FROM 
  cscart_categories as a 
  LEFT JOIN cscart_categories as b ON b.category_id IN (737) 
WHERE 
  a.id_path LIKE CONCAT(b.id_path, '/%')

Query time 0.00041

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
814
813
902
808
746
747
904
924
927
930
961
962
963
964
965
966
967
968
969
970
971
972
973
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009