Returning Default Values When Single Row Queries Fail

When performing SQL queries to retrieve specific data, it's common to encounter situations where no corresponding row exists. To avoid returning empty results, you may want to provide a default value.

Consider the following SQL statement that retrieves the next scheduled item for a stream:

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

If no matching row is found, this query will return an empty result. However, to ensure that something plays on the stream in case of no scheduled items, you can return a default value instead.

One approach to achieve this is by using the IFNULL or COALESCE function. By wrapping the original query within these functions, you can specify a default value that will be returned if no row is found:

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

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

However, these attempts will still result in an empty result when no rows are found. To address this issue, a more effective technique is to use an aggregate function like MIN along with IFNULL:

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

By using the aggregate function MIN, you ensure that you'll get a NULL result if no records are selected. This NULL value will then be replaced with the default value provided by IFNULL.