mysql Subquery error Unknown column 'tv.last_time' in 'where clause' - mysql

Question

When I use mysql to do subqueries, I get Unknown column 'tv.last_time' in 'where clause', what should I do? I want to use external conditions to filter out non-conforming records when doing subqueries
SELECT
info_topic.*, b.newMessage
FROM
info_topic
LEFT JOIN `info_topic_visit` AS tv ON tv.topic_id = info_topic.id
AND tv.user_id = 225
LEFT JOIN (
SELECT
p.topic_id,
count(*) AS newMessage
FROM
info_post p
WHERE
p.create_time > tv.last_time
GROUP BY
p.topic_id
) AS b ON b.topic_id = info_topic.id
ORDER BY b.newMessage DESC
database schema:
CREATE TABLE `info_topic` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(255) COLLATE utf8mb4_unicode_ci DEFAULT '' COMMENT 'topic name',
`summary` varchar(500) COLLATE utf8mb4_unicode_ci DEFAULT '' COMMENT 'summary`',
`create_time` datetime DEFAULT NULL COMMENT '',
`sort` int(11) DEFAULT '0' COMMENT '',
PRIMARY KEY (`id`),
);
CREATE TABLE `info_topic_visit` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`topic_id` int(11) DEFAULT '0' COMMENT '',
`user_id` int(11) DEFAULT '0' COMMENT '',
`last_time` datetime DEFAULT NULL COMMENT '',
PRIMARY KEY (`id`)
);

Answer 1

The problem with your subquery is that the tv alias has no meaning there, so you can't reference anything from the table to which that alias corresponds. One approach would be to instead convert your subquery into a correlated subquery in the select clause:
SELECT
i.*,
(SELECT COUNT(*) FROM info_post p
WHERE p.create_time > tv.last_time AND p.topic_id = i.id) AS newMessage
FROM info_topic i
LEFT JOIN info_topic_visit AS tv
ON tv.topic_id = info_topic.id AND tv.user_id = 225
LEFT JOIN info_topic_member tm
ON i.id = tm.topic_id AND
tm.del_flag = 0 AND
tm.apply_status = i.open_type AND
tm.user_id = 225
WHERE
i.del_flag = 0 AND
i.id > 0 AND
tm.id IS NOT NULL
ORDER BY
newMessage DESC;
There might also be a way to restructure your query so that you don't need to use correlated subqueries, which are inefficient.