mysql左连接顺序由空值到结尾

Question

MySQL的

SELECT * FROM media
LEFT JOIN media_priority
ON (media_priority.media_id = media.id AND media_priority.media_tag = '".$tag."')
WHERE something = 'something'
ORDER BY media_priority.media_order;

这工作正常,除了media_priority.media_order有时返回null并且mysql将空值放在顶部.

所以我试图弄清楚如何用我的左连接做一个IS NULL AS tempcol语句,所以我可以先按顺序排序.....我似乎无法正确使用语法.

我在哪里将IS NULL语句放在上面的查询中？

我想的是:

LEFT JOIN media_priority
ON (media_priority.media_id = media.id AND media_priority.media_tag = '".$tag."')
media_priority.media_order IS NULL AS isnull

但这不起作用.

Answer 1

ORDER BY 
    CASE WHEN media_priority.media_order IS NULL THEN 1 ELSE 0 END, 
    media_priority.media_order;

如果您有一些media_order永远无法达到的上限,请使用幻数.

ORDER BY 
    COALESCE( media_priority.media_order,99999999);

第一种方法显然更安全一些!

Answer 2

我建议先按布尔值media_priority.media_order IS NULLfirst 排序,然后按列值排序,以强制NULL结束.

像这样:

SELECT * FROM media
LEFT JOIN media_priority
ON (media_priority.media_id = media.id AND media_priority.media_tag = '".$tag."')
WHERE something = 'something'
ORDER BY (media_priority.media_order IS NULL) ASC, media_priority.media_order ASC;