SELECT 
  combination 
FROM 
  cscart_product_options_inventory 
WHERE 
  product_id = 478 
  AND amount > 0 
  AND combination != ''

Query time 0.00038

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE cscart_product_options_inventory range pc pc 770 13 Using index condition; Using where

Result

combination
968_19233
968_19234
968_19236
968_19237
968_19238
968_19239
968_19240
968_19241
968_19242
968_19244
968_19245
968_19246
968_19249
968_19250
968_19251
968_19252
968_23618
968_24306