如何减去存储在数据库中的变量

Question

我正在尝试从我存储的数据库中减去变量。试着运气却无法解决问题。所以我做了一个招聘流程，例如我为这个工作岗位头衔和工作类型开了2个空缺，然后当有人申请并且我雇用他/她时，空缺必须减去数据库中的一个。提前致谢！所以这是我的代码：

<?php
    if ($applied_position_title == $position_title && $applied_job_type == $job_type) {
        echo $vacancy;
        $output = --$vacancy;
        echo $output;
    }
    $query_status = "UPDATE job_opening SET closing_date = DATE('$newdate'), creator_id = '$creator_id', job_opening_status = 'Closed', vacancy = '$output' WHERE '$job_accepted' = 'Yes' AND '$applied_position_title' = position_title AND '$applied_job_type' = job_type";
    $result_status = mysqli_query($link, $query_status) or die(mysqli_error($link));
    echo $query_status;
?>

Answer 1

执行如下更新查询：

UPDATE `your_table` SET `filed_name` = (`filed_name` - 1) WHERE `some_id` = 1

Answer 2

在查询中尝试vacancy = (vacancy-1)

$query_status = "UPDATE job_opening SET closing_date = DATE('$newdate'), creator_id = '$creator_id', job_opening_status = 'Closed', vacancy = (vacancy-1) WHERE '$job_accepted' = 'Yes' AND '$applied_position_title' = position_title AND '$applied_job_type' = job_type";