Client Script for Web Client - Update column

I am having issues trying to understand how to update values using client scripts. My plan is to update non_part_usage.user_def4 to blank if the personID is null or empty.

My understanding is that setControlValue is used to update, but I am not sure how to link the value from the array (npu_id) and setControlValue.

Client Script

var personID = getControlValue('task','person_id');
alert(personID);

if(isNullOrEmptyString(personID))
{	
	var task_id = getControlValue('task','task_id');
	var npu_id = getDBValues('SP_COUNT_NPU',[task_id]);

	for(var i=0;i<size(npu_id);++i)	
	{	 	
		setControlValue('non_part_usage', 'user_def4', '');
	}
}

Client Script SQL 

SELECT npu_id FROM NON_PART_USAGE WHERE task_id =  '{0}'

A couple of questions:

1. How do you update the user_def4 to blank?
2. Is there a way to mass update all the values in the array?