Only Update a certain column in MySQL Query if it has a certain value set

Question

I have a Project Management application I am building with PHP and MySQL and the function below is used to mass update all Tasks associated with a project, in some cases this could be 200-300 MySQL records being updated at once.

I just realized there is a flaw in my design here though. This function UPDATES my database columns status , date_modified , and date_completed

This is great most of the time but I just realized that often a user will forget to "start" a Task which sets the date_started column.

If the date_started column is never set, it will have a value of 0000-00-00 00:00:00 and with a date_started column set to 0000-00-00 00:00:00 and the date_completed column set to NOW...this breaks my Gantt chart functionality for these Tasks.

So I need to modify my SQL below so that when setting a status to completed it will somehow check to see if the date_stared column === 0000-00-00 00:00:00 and if it does, it will need to also update that column to NOW.

I believe this is possible using CASE in MySQL instead of having to query each and every record into PHP to do the check and then do a separate UPDATE when needed but my experience is very little in these more advanced SQL features.

Could someone help me with a solution?

public function completeAllTasks($projectId, $completed_date = true){
    // Update Project Task STATUS and Date_MODIFIED
    $sql = "UPDATE  `project_tasks`
    SET  `status` =  'Completed',
            `modified_user_id` = '$this->user_id',
            `date_completed` = UTC_TIMESTAMP()
            `date_modified` = UTC_TIMESTAMP()
    WHERE  `project_id` = '$projectId'
    AND `status` != 'Completed'
    AND `heading` != '1'";

    return $this->db->query($sql);
}

---

UPDATE

I did some testing and this seems to do the trick...

`date_started` = (CASE
                    WHEN date_started = '0000-00-00 00:00:00'
                      THEN UTC_TIMESTAMP()
                      ELSE date_started
                END)

Answer 1

$sql = "UPDATE  `project_tasks`
SET  `status` =  'Completed',
      `modified_user_id` = '$this->user_id',
      `date_completed` = UTC_TIMESTAMP(),
      `date_modified` = UTC_TIMESTAMP(),
      `date_started` = CASE
          WHEN date_started = '0000-00-00 00:00:00' THEN NOW()
          ELSE date_started
       END
WHERE  `project_id` = '$projectId'
AND `status` != 'Completed'
AND `heading` != '1'";