select c.id, c.name, c.email, c.country_code, c.budget, c.used, a.last_audit_date
from customers as c
inner join (
select s.cid, s.audit_date as last_audit_date
from audits as s
where s.audit_date = (select max(audit_date) form audits where cid = s.cid)
) as a on c.id = a.cid