Nested query to reference field from parent query

April 8, 2022

Simplified example:

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

I need to reference the value from ticket_id instead of 1234

>Solution :

You may use table aliases here:

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;

Note that you might also be able to write your query without the correlated subquery, instead using joins:

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;