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SELECT 
  combination 
FROM 
  cscart_product_options_inventory 
WHERE 
  product_id = 526 
  AND amount > 0 
  AND combination != ''

Explain
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	cscart_product_options_inventory	range	pc	pc	770		5	Using index condition; Using where