您可以在此处使用表别名：

SELECT
    ticket_id,
    number AS `ticket number`,
    (SELECT COUNT(*)
     FROM ost_thread_entry ote
     INNER JOIN ost_thread ot ON ote.thread_id = ot.id
     WHERE ot.object_id = t.ticket_id) AS `number of posts in ticket`
FROM ost_ticket t;

请注意，您也可以在不使用相关子查询的情况下编写查询，而是使用联接：

SELECT
    t.ticket_id,
    t.number AS `ticket number`,
    COUNT(ote.thread_id) AS `number of posts in ticket`
FROM ost_ticket t
LEFT JOIN ost_thread ot ON ot.object_id = t.ticket_id
LEFT JOIN ost_thread_entry ote ON ote.thread_id = ot.id
GROUP BY
    t.ticket_id,
    t.number;