How to Perform a Left Join with Conditional Filtering in SQL?

SQL left join and conditional filtering

In database connections, it is often necessary to combine data from multiple tables while applying certain filtering conditions. Consider the following scenario:

You want to retrieve default settings from the settings table and, if needed, role-specific settings from the character_settings table for specific roles. However, the query below only returns the default if the role setting matches a specific value, and ignores the default if no role setting is defined:

SELECT `settings`.*, `character_settings`.`value`
FROM (`settings`)
LEFT JOIN `character_settings` 
ON `character_settings`.`setting_id` = `settings`.`id`
WHERE `character_settings`.`character_id` = '1'  

In order to retrieve all default settings, and get the role settings when available, we need to move the filter condition to the join clause:

SELECT  `settings`.*, `character_settings`.`value`
FROM    `settings`
LEFT JOIN 
       `character_settings` 
ON     `character_settings`.`setting_id` = `settings`.`id`
        AND `character_settings`.`character_id` = '1'  

By placing the WHERE clause in the join condition, we specify that only rows for which the left join succeeds will be included in the result set. This allows us to retrieve default settings while still accounting for the possibility of role-specific settings.

The above is the detailed content of How to Perform a Left Join with Conditional Filtering in SQL?. For more information, please follow other related articles on the PHP Chinese website!