如果未找到单行，则返回默认值

2021-11-20 00:00:00 default select mysql fallback null-coalescing

我有以下选择语句来获取流的下一个计划项目.如果没有匹配的行，我希望它返回一个默认值.这是我正在使用的 SQL:

I have the following select statement to grab the next scheduled item for a stream. If there is no matching row, I want it to return a default value. Here's the SQL that I'm using:

SELECT `file`
FROM `show`, `schedule` 
WHERE `channel` = 1
  AND `start_time` <= UNIX_TIMESTAMP() 
  AND `start_time` > UNIX_TIMESTAMP()-1800
  AND `show`.`id` = `schedule`.`file` 
ORDER BY `start_time`
DESC LIMIT 1

这应该抓取最近安排的项目，但如果它早于查询前 30 分钟，则不会.

That should grab the most recently scheduled item, but not if it's older than 30 minutes before the query.

但是，如果用户没有安排任何事情，我想要一个默认值，以便在流中实际播放某些内容.我尝试了以下方法:

However, if the user doesn't schedule anything, I want a default value, so that something actually plays on the stream. I've tried the following:

SELECT COALESCE(`file`, 'default.webm')
FROM `show`, `schedule`...

和

SELECT IFNULL(`file`, 'default.webm')
FROM `show`, `schedule`

然而，如果没有找到行，它总是返回一个空结果.我如何才能返回默认值?

However, it always returns an empty result if no rows are found. How can I return a default value instead?

推荐答案

一种方法

SELECT IFNULL(MIN(`file`), 'default.webm') `file` 
  FROM `show`, `schedule` 
 WHERE `channel` = 1 AND `start_time` <= UNIX_TIMESTAMP() 
   AND `start_time` > UNIX_TIMESTAMP()-1800 AND `show`.`id` = `schedule`.`file` 
 ORDER BY `start_time` DESC LIMIT 1

因为你只返回一行，你可以使用聚合函数，在这种情况下MIN()，确保你在没有选择任何记录时得到NULL.然后 IFNULL() 或 COALESCE() 将完成它的工作.

Since you return only one row, you can use an aggregate function, in that case MIN(), that ensures that you'll get NULL if no records selected. Then IFNULL() or COALESCE() will do its job.

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