Database: dog_kennels
Question: Find the zip code of the owner who spent the most money in total for his or her dogs.
SQL: select t1.zip_code from owners as t1 join dogs as t2 on t1.owner_id  =  t2.owner_id join treatments as t3 on t2.dog_id  =  t3.dog_id group by t1.owner_id order by sum(t3.cost_of_treatment) desc limit 1