嵌套查詢以引用父查詢中的字段

Question

簡化範例：

select
    ticket_id,
    `number` as 'ticket number',
    (SELECT count(*) from ost_thread_entry join ost_thread on ost_thread_entry.thread_id = ost_thread.id where ost_thread.object_id = 1234) as 'number of posts in ticket'
from
    ost_ticket

我需要引用 ticket_id 中的值，而不是 1234

Answer 1

您可以在此處使用表格別名：

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;

請注意，您也可以在不使用相關子查詢的情況下編寫查詢，而是使用聯結：

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;