generate json data with mysql help

Hi

This is the sample of data type i need to generate with 2 mysql queries

$obj = array(
				"attr" => array(
								"id" => 1,
								"rel" => "drive"					
							 ),
				"data" => "My Documents",
	
		"children" => array(
		array(
				"attr" => array(
								"id" => 2,
								"rel" => "file"
							),
				"data" => "file1.doc",
				"state" => ""
			 )
	)	
);
		
echo json_encode($obj); 

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this block should generate with

				"attr" => array(
								"id" => 1,
								"rel" => "drive"					
							 ),
				"data" => "My Documents",

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SELECT id,rel,data FROM tbl_drives ORDER BY id

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below block should generate with

		"children" => array(
		array(
				"attr" => array(
								"id" => 2,
								"rel" => "file"
							),
				"data" => "file1.doc",
				"state" => ""
			 )

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SELECT id,rel,data FROM tbl_files WHERE parent_id = '$id' ORDER BY id

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Any idea how to loop two queries to generate above json data?

Thanks