如果找不到行，则返回默认值

问题描述：

我有以下选择语句，以获取流的下一个预定项目.如果没有匹配的行，我希望它返回默认值.这是我正在使用的行:

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 line 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

那应该获取最近计划的项目，但如果它早于查询之前30分钟，则不会.但是，如果用户未安排任何时间，则我需要一个默认值，以使某些内容实际在流中播放.我尝试了以下方法:

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

并且:

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

但是，如果未找到任何行，它将始终返回空结果.我该如何返回默认值?

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

答

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

由于仅返回一行，因此可以使用聚合函数(在这种情况下为MIN())，该函数可确保在未选择任何记录的情况下获得NULL.然后IFNULL()或COALESCE()将完成其工作.

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.

如果找不到行，则返回默认值

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