Qual é o e-mail da parte que usou os serviços o maior número de vezes?
CREATE TABLE parties (party_email VARCHAR, party_id VARCHAR); CREATE TABLE party_services (customer_id VARCHAR)
SELECT t1.party_email FROM parties AS t1 JOIN party_services AS t2 ON t1.party_id = t2.customer_id GROUP BY t1.party_email ORDER BY COUNT(*) DESC LIMIT 1