Unknown column 'xxx' in 'field list' - mysql

Question

As sometimes happens, a block of code fails and no matter how hard you try, you can't figure out where it is. In these cases a second pair of eyes sometimes sees what the brain doesn't register. I think this is one of those cases. It's almost certainly my fault and I did something wrong but I honestly can't figure out where.
This is a SELECT I wrote
SELECT
`a.dev_act_id`,
`a.dev_act_code`,
`a.dev_act_desc`,
`a.dev_act_type`,
`a.lang_code`,
`pa.dev_plan_act_id`,
`pa.action_status`,
`pa.action_expiration`,
`cb.competence_id`,
`cb.credits` AS `avail_credits`,
`w.credits` AS `settled_credits`
FROM
`pbq_idp_plan_actions` AS pa
INNER JOIN `pbq_idp_dev_actions` AS a ON `pa.dev_act_id` = `a.dev_act_id`
INNER JOIN `pbq_idp_credit_bags` AS cb ON `pa.dev_plan_act_id` = `cb.dev_plan_act_id`
INNER JOIN `pbq_idp_wallets` AS w ON `a.dev_act_id` = `w.dev_act_id`
WHERE
`pa.dev_plan_id` = 0
ORDER BY
`cb.competence_id`
The structure of the pbq_idp_dev_actions table whose alias is 'a', is as follows
CREATE TABLE `pbq_idp_dev_actions` (
`dev_act_id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
`wallet_id` bigint(20) unsigned NOT NULL,
`dev_act_code` text COLLATE utf8mb4_unicode_520_ci NOT NULL,
`dev_act_desc` longtext COLLATE utf8mb4_unicode_520_ci NOT NULL,
`dev_act_type` tinyint(1) unsigned NOT NULL DEFAULT '0',
`lang_code` varchar(7) COLLATE utf8mb4_unicode_520_ci NOT NULL,
PRIMARY KEY (`dev_act_id`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_520_ci
It would seem that everything is fine, but I get the following error and I don't understand why. The alias is correct, the field exists, yet the system can't find it.
WordPress database error: [Unknown column 'a.dev_act_id' in 'field list']
By request, here are the other three tables:
CREATE TABLE `pbq_idp_plan_actions` (
`dev_plan_act_id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
`dev_act_id` bigint(20) unsigned NOT NULL,
`dev_plan_id` bigint(20) unsigned NOT NULL,
`action_status` tinyint(2) unsigned NOT NULL DEFAULT '0',
`not_earlier` datetime DEFAULT NULL,
`deadline` datetime DEFAULT NULL,
PRIMARY KEY (`dev_plan_act_id`),
KEY `dev_act_id` (`dev_act_id`,`dev_plan_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_520_ci
CREATE TABLE `pbq_idp_credit_bags` (
`credit_bag_id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
`dev_plan_act_id` bigint(20) unsigned NOT NULL,
`competence_id` varchar(4) COLLATE utf8mb4_unicode_520_ci NOT NULL,
`credits` tinyint(3) NOT NULL DEFAULT '0',
PRIMARY KEY (`credit_bag_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_520_ci
CREATE TABLE `pbq_idp_wallets` (
`wallet_id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
`dev_act_id` bigint(20) unsigned NOT NULL,
`competence_id` varchar(4) COLLATE utf8mb4_unicode_520_ci NOT NULL,
`credits` tinyint(3) NOT NULL DEFAULT '0',
PRIMARY KEY (`wallet_id`),
KEY `dev_act_id` (`dev_act_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_520_ci

Answer 1

The problem is in the wrong usage of backticks as can be seen in this example and the column action_expiration doesn't exist on pbq_idp_plan_actions table.
Backticks are used in MySQL to select Schema Object Names. Do not put the alias name inside backticks.
Not valid
`a.dev_act_id`
Valid
a.`dev_act_id`.
In your case backticks are exces