MySQL Left Join，如果为null，则从一个或另一个中选择一个字段

Question

I am trying to LEFT JOIN 2 tables. which is working out fine. But i am getting back two sets of fields named setting_value . iam trying to get tblSettings . setting_value only if tblAgencySettings . setting_value is NULL. How would i go about doing this? I know i can rename the fields, then in PHP i can check the tblAgencySettings . setting_value and if NULL then grab the tblSettings . setting_value but i prefer to keep this at MySQL.

SELECT `tblSettings`.`id`, `tblSettings`.`setting_name`,
       `tblSettings`.`setting_value`, `tblAgencySettings`.`setting_value`
FROM `tblSettings` LEFT JOIN `tblAgencySettings` 
ON `tblSettings`.`id` = `tblAgencySettings`.`setting_id`
AND `tblAgencySettings`.`agency_id` = '1'
WHERE `tblSettings`.`changeable` = '1'

slight issue i just noticed. i failed to mention this. if tblAgencySettings.setting_value does have a value. but changeable is not 1 then just select tblSettings.setting_value

Answer 1

Just add a COALESCE :

SELECT `tblSettings`.`id`, `tblSettings`.`setting_name`,
       COALESCE(`tblAgencySettings`.`setting_value`, `tblSettings`.`setting_value`)
FROM `tblSettings` LEFT JOIN `tblAgencySettings` 
ON `tblSettings`.`id` = `tblAgencySettings`.`setting_id`
AND `tblAgencySettings`.`agency_id` = '1'
WHERE `tblSettings`.`changeable` = '1'

The COALESCE function returns the first non-NULL value you give it so this:

COALESCE(`tblAgencySettings`.`setting_value`, `tblSettings`.`setting_value`)

Will be tblAgencySettings.setting_value if that's not NULL and tblSettings.setting_value if tblAgencySettings.setting_value is NULL.

If tblAgencySettings.setting_value can also be zero and you want to ignore that as well as NULL, then you could use this instead of the COALESCE above:

COALESCE(
    IF(`tblAgencySettings`.`setting_value` = 0, NULL, `tblAgencySettings`.`setting_value`),
   `tblSettings`.`setting_value`
)

The IF returns the second argument if the first is true and the third if the first argument is false so the above use converts zero to NULL. Or, you could go all the way to a CASE statement:

case
    when `tblAgencySettings`.`setting_value` = 0     then `tblSettings`.`setting_value`
    when `tblAgencySettings`.`setting_value` IS NULL then `tblSettings`.`setting_value`
    else `tblSettings`.`setting_value`
end    

Answer 2

Change your SQL Statement to this:

SELECT `tblSettings`.`id`, `tblSettings`.`setting_name`, 
   CASE WHEN `tblSettings`.`setting_value` IS NULL THEN `tblAgencySettings`.`setting_value` 
    ELSE `tblSettings`.`setting_value` END AS `setting_value` 
    FROM `tblSettings` LEFT JOIN `tblAgencySettings` 
    ON `tblSettings`.`id` = `tblAgencySettings`.`setting_id`
    AND `tblAgencySettings`.`agency_id` = '1'
    WHERE `tblSettings`.`changeable` = '1'

Here's a link to MYSQL CASE Statement for your reference.