Question:

How can I create a report to show resolved tickets with only one agent replying to the ticket?

Answer:

This will show resolved tickets ordered by the number of agents who have added replies (or notes).

SELECT DPQL_COUNT_DISTINCT(tickets_messages.person) AS 'Number of agents', tickets_messages.ticket_id
FROM tickets_messages
WHERE tickets_messages.person.is_agent AND tickets_messages.ticket.status = 'resolved'
GROUP BY tickets_messages.ticket_id
ORDER BY DPQL_COUNT_DISTINCT(tickets_messages.person)