This query will give you the previous months tickets opened by category.

SELECT 
 COUNT(HD_TICKET.ID) AS NUMBER_TICKETS,
 HD_CATEGORY.NAME AS CATEGORY,
 date_format(HD_TICKET.CREATED, '%M %Y') AS DATE_CREATED
FROM
 HD_TICKET
 JOIN
 HD_CATEGORY ON (HD_CATEGORY.ID = HD_TICKET.HD_CATEGORY_ID)
WHERE
 (HD_TICKET.HD_QUEUE_ID = 1)
 AND (TIMESTAMP(HD_TICKET.CREATED) BETWEEN date_format(NOW() - INTERVAL 1 MONTH, '%Y-%m-01') 
 AND last_day(NOW() - INTERVAL 1 MONTH))
GROUP BY DATE_CREATED , CATEGORY

The results look like this.