Getting check box values through query string and saving them into database.

I am looking ofr some code revision please, I am trying to save/update data from some check boxes ( based on groups and sub groups. Everything is working perfect just have one little glitch that I can't figure out. when I chance selection (EDIT check boxes) how do I minipulate the change and save it. Origionally how I did was, I deleted everything with the Contact ID and re-saved fresh data but that messes things up.

Question would be how do I clear check boxes data when user checks un-checks data. The old data is saved in database. ( i have three tables. groups, sub-groups and a look up table group-subgrups with contact ids)
Any ideas/suggesions?

I have attached the screenshot just to get an idea how groups/sub-groups are being displayed.

I have attached the code for save ( that works perfectly )
and update ( that's where I have some issues )
 Thanks.

Save groups/subgroups
--------------------
//pass to record set
$rs = mysql_query($sql, $cn);
while($row = mysql_fetch_array($rs))
{
print "<p>";
$group_id= $row["group_id"] ;
	
//name='maingroup[]' value="new_group_is_here___3"
//the part before the "___" is the actual group name;
//the part after the "___" is the group id
$group_name = $row["group_name"];
$group_value = preg_replace('/\s/','_',$group_name) . "___" . $group_id;
//$group_name_array[] = $group_name;
 
// if had check box for the Main Groups
Print " <input type='hidden' name='maingroup[]' value='" . $group_value ."'/>" . "  " . "<strong>". $group_name . "</strong>: "."</ br>";
	
//sub group part - start
$sql2 = "Select * from tbl_sub_groups where group_id = '$group_id'";
 
//pass to record set
$rs2 = mysql_query($sql2, $cn);
 
// check the number of records and print the main group name or skip it if not children.
	while($row2 = mysql_fetch_array($rs2))
	{
	$id= $row2["sub_group_id"] ;
	$sgroup_name = $row2["sgroup_name"];
 
	print "</ br>";
	Print " <input type='checkbox' name='{$group_value}[]' 
               value='{$id}'/>" . "  " . $sgroup_name . "</ br>";
	}
	// sub group part ends.
 	print "</p>";                                   
}
 
	
$maingroup=$_REQUEST['maingroup'];
foreach($maingroup as $v){
 
	list($group_name,$group_id)=explode("___",$v);
	$group_id = preg_replace('/_/',' ', $group_id);
 
	$subgroup=$_REQUEST[$v];
 
	if($subgroup){
		foreach($subgroup as $v){
		
	   	 $sql3 = "INSERT INTO `tbl_people_groups` (`people_id`, `group_id`, `sub_group_id`) 
                                         VALUES ('$people_id', '$group_id', '$v')";
	   	 $rs3 = mysql_query($sql3, $cn);
		 }	
		}
		else{
			    $sql3 = "INSERT INTO `tbl_people_groups` (`people_id`, `group_id`, `sub_group_id`) 
                                         VALUES ('$people_id', '$group_id', '')";
				$rs3 = mysql_query($sql3, $cn);
		}								 
}
 
	
$maingroup=$_REQUEST['maingroup'];
foreach($maingroup as $v){
 
	list($group_name,$group_id)=explode("___",$v);
	$group_id = preg_replace('/_/',' ', $group_id);
 
	$subgroup=$_REQUEST[$v];
 
	if($subgroup){
		foreach($subgroup as $v){
		
	   	 $sql3 = "INSERT INTO `tbl_people_groups` (`people_id`, `group_id`, `sub_group_id`) 
                                         VALUES ('$people_id', '$group_id', '$v')";
	   	 $rs3 = mysql_query($sql3, $cn);
		 }	
		}
		else{
			    $sql3 = "INSERT INTO `tbl_people_groups` (`people_id`, `group_id`, `sub_group_id`) 
                                         VALUES ('$people_id', '$group_id', '')";
				$rs3 = mysql_query($sql3, $cn);
		}								 
}
 
 
Edit:
---------------
//pass to record set
$rs = mysql_query($sql, $cn);
while($row = mysql_fetch_array($rs))
{
	print "<p>";
  
	$group_id= $row["group_id"] ;
	
	//name='maingroup[]' value="new_group_is_here___3"
	//the part before the "___" is the actual group name;
	//the part after the "___" is the group id
	$group_name = $row["group_name"];
	$group_value = preg_replace('/\s/','_',$group_name) . "___" . $group_id;
	//$group_name_array[] = $group_name;
	
    if (in_array(trim($group_id), $selected_groups))
{
   $checkedString = "checked=\"checked\"";
}
else
{
   $checkedString = "";
}
 
 
print "<input type='hidden' name='maingroup[]' value='" . $group_value ."' $checkedString />" . "  " . "<strong>". $group_name . "</strong>: "."</ br>";
 
	//sub group part - start
	$sql2 = "Select * from tbl_sub_groups where group_id = '$group_id'";
 
	//pass to record set
	$rs2 = mysql_query($sql2, $cn);
	while($row2 = mysql_fetch_array($rs2))
	{
		$sgroup_id= $row2["sub_group_id"] ;
		$sgroup_name = $row2["sgroup_name"];
		$group_id = $row2["group_id"];
		 
		$sql4 =  "Select * from tbl_people_groups where people_id = '$id' and group_id = '$group_id'";
        $rs4 = mysql_query($sql4, $cn);
	//echo $sql4;
	    while($row4 = mysql_fetch_array($rs4))
	    {
		 $selected_sub_groups[]= $row4["sub_group_id"];
	    }
 
	
	 	   if (in_array(trim($sgroup_id), $selected_sub_groups))
			{
			//echo "test 1";
 			  $checkedString2 = "checked=\"checked\"";		
			  print "</ br>";
		      Print " <input type='checkbox' name='{$group_value}[]' value='{$sgroup_id}' checked='$checkedString2'/>" . "  " . $sgroup_name . "</ br>";
			}
			else
			{ 
			//echo "test 2";
  			 $checkedString2 = "";
	
			  print "</ br>";
		      Print " <input type='checkbox' name='{$group_value}[]' value='{$sgroup_id}' />" . "  " . $sgroup_name . "</ br>";
			  
			}
 
	}
	// sub group part ends.
	print "</p>";                                   
}
 
Save:
-------
foreach($_REQUEST['maingroup'] as $grp){
// echo "Group:" . $grp . "<br />";
$pizza  = "Media___12";
$pieces = explode("___", $pizza);
// $pieces[0]. "<br />"; // piece1
$group = $pieces[1];
 
 $sql3 = "DELETE  from tbl_people_groups where people_id = '$id'";
  $rs = mysql_query($sql3,$cn);
  
 
 //explode("___", $grp)
  foreach( $_REQUEST[$grp] as $subgrp )
  {
  
 $sql2 = "insert into tbl_people_groups(people_id, group_id, sub_group_id) 
	                            values('$id','$group','$subgrp')";
		
  $rs = mysql_query($sql2,$cn);
		
// echo "Subgroup:" . $subgrp;
  }
}

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