How to Return Default Values in SQL When No Matching Rows are Found?

Returning Default Values with SQL When Single Rows Are Not Found

When retrieving data from a database using a SQL query, it's often desirable to return a default value if no matching row is found. This is especially useful when dealing with situations where the query output is intended for display or processing in applications.

Original Query

Consider the following SQL query, which aims to retrieve the next scheduled item for a stream:

<code class="sql">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</code>

This query returns the most recent scheduled item that meets the specified criteria. However, if no matching row is found (i.e., there are no scheduled items within the last 30 minutes), an empty result is returned.

Default Value Approaches

To handle this scenario and return a default value when no matching row is found, several approaches can be used:

COALESCE/IFNULL with Aggregation

One method involves using the COALESCE or IFNULL functions in conjunction with an aggregate function. This approach ensures that a non-null value is returned even if the query result is empty. For example:

<code class="sql">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</code>

In this query, the MIN() aggregate function is used to return the minimum value for the file column. This ensures that a NULL value is returned if no rows are selected. The IFNULL or COALESCE function then replaces the NULL value with the default value 'default.webm'.

This solution effectively returns the default value when no matching row is found, allowing the application to handle the absence of scheduled items appropriately.

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