SELECT 
  combination 
FROM 
  cscart_product_options_inventory 
WHERE 
  product_id = 10219 
  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 54 Using index condition; Using where

Result

combination
643_15362_644_15367
643_15363_644_15367
643_15364_644_15367
643_15365_644_15367
643_15366_644_15367