SELECT `id` , ( SELECT `id` FROM `rf_content` WHERE `channel` = `rf_kankan`.`id` AND `status` = 3 ORDER BY `publish_time` DESC LIMIT 1 ) AS `latest_content_id` FROM `rf_kankan` WHERE `id` IN (27, 29, 110, 113) ORDER BY ( SELECT `publish_time` FROM `rf_content` WHERE `channel` = `rf_kankan`.`id` AND `status` = 3 ORDER BY `publish_time` DESC LIMIT 1 ) DESC LIMIT 5 优化下

可以考虑使用子查询转为 JOIN 的方式优化这个 SQL，如下所示：

SELECT k.`id`, c.`id` AS `latest_content_id`
FROM `rf_kankan` k
LEFT JOIN (
    SELECT `channel`, MAX(`publish_time`) AS `latest_publish_time`, MAX(`id`) AS `id`
    FROM `rf_content`
    WHERE `status` = 3
    GROUP BY `channel`
) c ON k.`id` = c.`channel`
WHERE k.`id` IN (27, 29, 110, 113)
ORDER BY c.`latest_publish_time` DESC
LIMIT 5;

这个优化的思路是先使用子查询获取每个 `channel` 下最新的 `publish_time` 和对应的 `id`，然后使用 LEFT JOIN 将 `rf_kankan` 和 `rf_content` 表关联起来，得到每个 `channel` 对应的最新的 `id`。最后按照 `latest_publish_time` 排序，取前 5 条结果。

这个优化的好处是只需要查询一次 `rf_content` 表，减少了子查询的使用，可以提高查询效率。

相关问题

优化sql语句update jc_content set views_day=(select contentcou1_.views_day from jc_content_count contentcou1_ where jc_content.content_id=contentcou1_.content_id), comments_day=(select contentcou2_.comments_day from jc_content_count contentcou2_ where jc_content.content_id=contentcou2_.content_id), downloads_day=(select contentcou3_.downloads_day from jc_content_count contentcou3_ where jc_content.content_id=contentcou3_.content_id), ups_day=(select contentcou4_.ups_day from jc_content_count contentcou4_ where jc_content.content_id=contentcou4_.content_id)

可以将子查询改为连接查询，减少查询次数，提高效率：

UPDATE jc_content 
INNER JOIN jc_content_count ON jc_content.content_id = jc_content_count.content_id 
SET jc_content.views_day = jc_content_count.views_day,
    jc_content.comments_day = jc_content_count.comments_day,
    jc_content.downloads_day = jc_content_count.downloads_day,
    jc_content.ups_day = jc_content_count.ups_day;

同时，可以对 `jc_content` 和 `jc_content_count` 表的 `content_id` 字段建立索引，加快连接查询的速度。

SELECT cbdl_code, CASE WHEN LENGTH(cbdl_code) = 1 THEN name WHEN LENGTH(cbdl_code) = 3 THEN CONCAT_WS('/', (SELECT name FROM pms_cbdl WHERE cbdl_code = SUBSTRING(t1.cbdl_code, 1, 1)), (SELECT name FROM pms_cbdl WHERE cbdl_code = t1.cbdl_code)) WHEN LENGTH(cbdl_code) = 4 THEN CONCAT_WS('/', (SELECT name FROM pms_cbdl WHERE cbdl_code = SUBSTRING(t1.cbdl_code, 1, 1)), (SELECT name FROM pms_cbdl WHERE cbdl_code = SUBSTRING(t1.cbdl_code, 1, 3)), (SELECT name FROM pms_cbdl WHERE cbdl_code = t1.cbdl_code)) WHEN LENGTH(cbdl_code) = 5 THEN CONCAT_WS('/', (SELECT name FROM pms_cbdl WHERE cbdl_code = SUBSTRING(t1.cbdl_code, 1, 1)), (SELECT name FROM pms_cbdl WHERE cbdl_code = SUBSTRING(t1.cbdl_code, 1, 3)), (SELECT name FROM pms_cbdl WHERE cbdl_code = SUBSTRING(t1.cbdl_code, 1, 4)), (SELECT name FROM pms_cbdl WHERE cbdl_code = t1.cbdl_code)) END AS new_name FROM pms_cbdl t1;以上语句执行报错

您好！根据您提供的SQL语句 SELECT cbdl_code, CASE WHEN LENGTH(cbdl_code) = 1 THEN name WHEN LENGTH(cbdl_code) = 3 THEN CONCAT_WS('/', (SELECT name FROM pms_cbdl WHERE cbdl_code = SUBSTRING(t1.cbdl_code, 1, 1)), (SELECT name FROM pms_cbdl WHERE cbdl_code = t1.cbdl_code)) WHEN LENGTH(cbdl_code) = 4 THEN CONCAT_WS('/', (SELECT name FROM pms_cbdl WHERE cbdl_code = SUBSTRING(t1.cbdl_code, 1, 1)), (SELECT name FROM pms_cbdl WHERE cbdl_code = SUBSTRING(t1.cbdl_code, 1, 3)), (SELECT name FROM pms_cbdl WHERE cbdl_code = t1.cbdl_code)) WHEN LENGTH(cbdl_code) = 5 THEN CONCAT_WS('/', (SELECT name FROM pms_cbdl WHERE cbdl_code = SUBSTRING(t1.cbdl_code, 1, 1)), (SELECT name FROM pms_cbdl WHERE cbdl_code = SUBSTRING(t1.cbdl_code, 1, 3)), (SELECT name FROM pms_cbdl WHERE cbdl_code = SUBSTRING(t1.cbdl_code, 1, 4)), (SELECT name FROM pms_cbdl WHERE cbdl_code = t1.cbdl_code)) END AS new_name FROM pms_cbdl t1; ，由于缺少表结构和完整的错误信息，无法准确判断报错原因。

请提供完整的错误信息，以及相关的表结构和数据，这样我才能更好地帮助您解决问题。谢谢！