SELECT 
  combination 
FROM 
  cscart_product_options_inventory 
WHERE 
  product_id = 514 
  AND amount > 0 
  AND combination != ''

Query time 0.00049

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE cscart_product_options_inventory range pc pc 770 23 Using index condition; Using where

Result

combination
584_14715_727_16015
584_14716_727_16015
584_14717_727_16015
584_14718_727_16015