Question

我正在尝试LEFT JOIN 2表。这很好。但我回来了两组名为setting_value的字段。我只是在tblSettings。setting_value为NULL时尝试获取tblAgencySettings。setting_value。我该怎么做呢？我知道我可以重命名字段，然后在PHP中我可以检查tblAgencySettings。setting_value如果为NULL则抓住tblSettings。setting_value但是我更喜欢保留它MySQL的。

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'

我刚注意到的一个小问题。我没提到这个。如果tblAgencySettings.setting_value确实有值。但是更改不是1然后只需选择tblSettings.setting_value

Answer 1

只需添加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'

COALESCE函数返回你给它的第一个非NULL值，所以：

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

如果tblAgencySettings.setting_value不为NULL，则为tblSettings.setting_value;如果tblAgencySettings.setting_value为NULL，则为tblAgencySettings.setting_value。

如果COALESCE( IF(`tblAgencySettings`.`setting_value` = 0, NULL, `tblAgencySettings`.`setting_value`), `tblSettings`.`setting_value` )也可以为零，并且您想忽略它以及NULL，那么您可以使用它而不是上面的COALESCE：

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

如果第一个参数为真，则IF返回第二个参数，如果第一个参数为假，则返回第三个参数，因此上述使用将零转换为NULL。或者，您可以一直到CASE语句：

{{1}}

Answer 2

将您的SQL语句更改为：

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.

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

2 个答案: