SELECT 
  combination 
FROM 
  cscart_product_options_inventory 
WHERE 
  product_id = 426 
  AND amount > 0 
  AND combination != ''

Query time 0.00062

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE cscart_product_options_inventory range pc pc 770 31 Using index condition; Using where

Result

combination
1365_24304_614_15104_696_15775
1365_24304_614_15104_696_15776
1365_24304_614_15105_696_15775
1365_24304_614_15106_696_15775
1365_24304_614_15106_696_15776
1365_24304_614_15107_696_15775
1365_24304_614_15107_696_15776
1365_24304_614_15108_696_15775
1365_24304_614_15108_696_15776
1365_24304_614_15109_696_15775
1365_24304_614_15109_696_15776
1365_24304_614_15110_696_15775
1365_24304_614_15111_696_15775
1365_24304_614_15112_696_15775
1365_24304_614_15112_696_15776
1365_24304_614_15113_696_15775
1365_24304_614_15113_696_15776
1365_24304_614_15114_696_15775
1365_24304_614_15114_696_15776
1365_24304_614_15115_696_15775
1365_24304_614_15553_696_15775
1365_24304_614_15554_696_15775
1365_24304_614_23711_696_15775
1365_24304_614_23711_696_15776
1365_24305_614_15104_696_15775
1365_24305_614_15104_696_15776
1365_24305_614_15105_696_15775
1365_24305_614_15105_696_15776
1365_24305_614_15107_696_15775
1365_24305_614_15107_696_15776
1365_24305_614_15114_696_15775
1365_24305_614_15114_696_15776
1365_24305_614_15115_696_15775
1365_24305_614_15115_696_15776
1365_24305_614_15553_696_15775
1365_24305_614_15553_696_15776
1365_24305_614_15554_696_15775
1365_24305_614_15554_696_15776
1365_24305_614_23711_696_15775
1365_24305_614_23711_696_15776