获取MYSQL中某列为空的表中的值

Question

Video
-------
id
source_low
source_med
source_high

Source
--------
id
duration
thumbnail

视频项可以包含至少 1 个或最多 3 个源（低源、中源、高源）。

这个想法与 youtube 相同，这意味着视频记录可以包含多种质量。因此，当我返回持续时间/缩略图时，我想检查哪个源存在并返回其中一个。

例如，如果视频有 source_low 或 source_med ，这意味着 source_low 和 source_med 有持续时间和缩略图，那么我只需要返回其中一个。

Codeigniter 语法：

  $this->db->select('v.*, sl.duration, sm.duration as duration_m, sh.duration as duration_h, sl.thumbnail, sm.thumbnail as thumbnail_m, sh.thumbnail as thumbnail_h');
        $this->db->from('video as v');
        $this->db->join('source as sl', 'v.source_low = sl.video_id', 'left');
        $this->db->join('source as sm', 'v.source_med = sm.video_id', 'left');
        $this->db->join('source as sh', 'v.source_high = sh.video_id', 'left');
        $this->db->group_by('v.id');

SQL（缩略图获取方式相同）：

SELECT v.*, sl.duration, sm.duration as duration_sm, sh.duration as duration_sh
    FROM video as v
    LEFT JOIN source as sl ON (v.source_low = sl.video_id)
    LEFT JOIN source as sm ON (v.source_med = sm.video_id)
    LEFT JOIN source as sh ON (v.source_high = sh.video_id)
    GROUP BY v.id;

现在我可以返回所有的结果，包括空值，但是我怎样才能只返回一个持续时间和一个缩略图呢？

感谢您的帮助

更新：

只需更新查询，但似乎 codeigniter 在处理它时出现了一些错误， 任何想法？谢谢

我的代码：

$this->db->select('v.*, COALESCE(b.duration, bt.duration, bc.duration) AS duration, COALESCE(b.thumbnail, bt.thumbnail, bc.thumbnail) AS thumbnail');

在 SQL 中：

Error Number: 1064

You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'AS duration, COALESCE(b.thumbnail, `bt`.`thumbnail`, `bc`.`thumbnail)` AS thumbn' at line 1

SELECT `v`.*, `t`.`name`, `t`.`name_tw`, `t`.`name_cn`, COALESCE(b.duration, `bt`.`duration`, `bc`.`duration)` AS duration, COALESCE(b.thumbnail, `bt`.`thumbnail`, `bc`.`thumbnail)` AS thumbnail FROM (`video` as v) JOIN `teacher` as t ON `v`.`teacher_id` = `t`.`id` LEFT JOIN `brightcove` as b ON `v`.`video` = `b`.`video_id` LEFT JOIN `brightcove` as bt ON `v`.`video_tw` = `bt`.`video_id` LEFT JOIN `brightcove` as bc ON `v`.`video_cn` = `bc`.`video_id` WHERE `v`.`is_delete` = 0 AND `v`.`type` = 0 GROUP BY `v`.`id` ORDER BY `v`.`start_date` desc

Answer 1

您可以使用COALESCE()function 返回第一个非空值。有关详细信息，请参阅 MySQL manual。例如：

SELECT v.*, COALESCE(sl.duration, sm.duration, sh.duration) AS duration
    FROM video as v
    LEFT JOIN source as sl ON (v.source_low = sl.video_id)
    LEFT JOIN source as sm ON (v.source_med = sm.video_id)
    LEFT JOIN source as sh ON (v.source_high = sh.video_id)
    GROUP BY v.id;