I Suggest to change repeater's header to general name for fruit and vegetable
like productid, produtname, varedi
then your select statement will be:
SELECT frugt_id AS Product_id, frugt_navn AS ProductName, vaerdi AS vaerdi, FROM frugt UNION SELECT groent_id AS ProductID, groent_navn AS ProductName, groent_vaerdi AS Varedi FROM groent
then you will not have 0 value in your repeater.
Hope it helps