How to Return a Default Value for Absent SQL Rows When Using Joins?

How to Return a Default Value for Absent SQL Rows

Suppose you have an active query that retrieves information from two joined tables, show and schedule, based on specific criteria. If there are no matching rows in your query, how do you ensure that a default value is returned instead of an empty result?

One solution is to utilize the IFNULL() or COALESCE() function in your query. These functions take two arguments:

IFNULL(expression, default_value)
COALESCE(expression, default_value)

expression refers to the value you are checking for nullity, and default_value is the value you want to return if expression is null.

Example:

<code class="sql">SELECT IFNULL(`file`, 'default.webm') AS `file`
FROM `show`, `schedule`...</code>

This query will return 'default.webm' if there are no matching rows.

However, in your specific case, where you are retrieving only a single row, you can leverage an aggregate function to ensure a null value in case of no matching rows:

<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>

The use of MIN() ensures that a null value is returned if no records are selected. Subsequently, IFNULL() will then replace the null value with the default value, ensuring that your query always returns a non-empty result.

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