SELECT 
  combination 
FROM 
  cscart_product_options_inventory 
WHERE 
  product_id = 9938 
  AND amount > 0 
  AND combination != ''

Query time 0.00112

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE cscart_product_options_inventory range pc pc 770 8 Using index condition; Using where

Result

combination
699_15792
699_15793
699_15794
699_15795
699_15796
699_15797
699_15798
699_15799
699_15800
699_15801
699_15802
699_15803
699_15804
699_15805
699_15806
699_15807