Mysql 在Codeigniter中使用查询生成器-如何正确使用别名？

我使用codeigniter 3.x中的查询生成器构建了以下查询

        $this->db->select('
            categories.*,
            posts.id,
            posts.category_id,
            posts.user_id,
            posts.updated_user_id,
            posts.title,
            posts.slug,
            posts.body,
            posts.post_image,
            year(posts.created_timestamp) as year, 
            month(posts.created_timestamp) as month
        ');

上述操作会在“where子句”中生成错误的未知列“year”。我已经转储了sql，但无法理解为什么sql不能识别年份和月份

SELECT 
    `categories`.*, 
    `posts`.`id`, 
    `posts`.`category_id`, 
    `posts`.`user_id`, 
    `posts`.`updated_user_id`, 
    `posts`.`title`, 
    `posts`.`slug`, 
    `posts`.`body`, 
    `posts`.`post_image`, 
     year(posts.created_timestamp) as year, 
     month(posts.created_timestamp) as month 
     FROM `posts` 
     JOIN `categories` ON `categories`.`id` = `posts`.`category_id` 
     WHERE 
        `year` = '2019' 
     AND `month` = '1' 
     LIMIT 5

---

这不是codeigniter的问题，MySQL的工作原理不是字段名（请避免使用这种字段名） 它是一个函数，选择或调用函数与定义函数不同，所以必须以选择该函数的相同方式放置where子句

SELECT 
    `categories`.*, 
    `posts`.`id`, 
    `posts`.`category_id`, 
    `posts`.`user_id`, 
    `posts`.`updated_user_id`, 
    `posts`.`title`, 
    `posts`.`slug`, 
    `posts`.`body`, 
    `posts`.`post_image`, 
     year(posts.created_timestamp) as year, 
     month(posts.created_timestamp) as month 
     FROM `posts` 
     JOIN `categories` ON `categories`.`id` = `posts`.`category_id` 
     WHERE 
        year(posts.created_timestamp) = '2019' 
     AND month(posts.created_timestamp) = '1' 
     LIMIT 5