IF ELSEIF ทำไมเงื่อนไขเข้าแค่เงื่อนไขเดียว รบกวนช่วยดูทีครับ

<?php
include 'connect.php';

$day = $_POST['day'];
$month = $_POST['month'];
$year = $_POST['year'];

if ($year != "")
{
	$data = $db->prepare('SELECT YEAR(start)+543 AS dtYear, COUNT((YEAR(start))+543) AS count_year
                          FROM tbl_logs
                          WHERE YEAR(start) = :y
                          GROUP BY dtYear
                          HAVING count_year
                          ORDER BY dtYear DESC');
	$data->bindParam(':y',$year);
	$data->execute();

	while ($rows = $data->fetch(PDO::FETCH_ASSOC)) 
		{
			echo $rows['dtYear'].'-'.$rows['count_year'];
		}
}

 
	
else if ($month != "" && $year != "") 
{
	$data1 = $db->prepare('SELECT YEAR(start)+543 AS dtYear, COUNT((YEAR(start))+543) AS count_year
                          FROM tbl_logs
                          WHERE YEAR(start) = :y AND MONTH(start) = :m
                          GROUP BY dtYear
                          HAVING count_year
                          ORDER BY dtYear DESC');
	$data1->bindParam(':y',$year);
	$data1->bindParam(':m',$month);
	$data1->execute();

	while ($rows1 = $data1->fetch(PDO::FETCH_ASSOC)) 
		{
			echo $rows1['dtYear'].'-'.$rows1['count_year'];
		}
}


else if($day !="" && $month !="" && $year != "") 
{
	$data1 = $db->prepare('SELECT YEAR(start)+543 AS dtYear, COUNT((YEAR(start))+543) AS count_year
                          FROM tbl_logs
                          WHERE YEAR(start) = :y AND MONTH(start) = :m
                          GROUP BY dtYear
                          HAVING count_year
                          ORDER BY dtYear DESC');
	$data1->bindParam(':y',$year);
	$data1->bindParam(':m',$month);
	$data1->execute();

	while ($rows1 = $data1->fetch(PDO::FETCH_ASSOC)) 
		{
			echo $rows1['dtYear'].'-'.$rows1['count_year'];
		}
}
?>

if( $year && $month){
    // ทำ  ถ้า year มีค่า และ month มีค่า
}else if( $year){
    // ทำ ถ้า year มีค่า และ month ไม่มีค่า
}else if( $month){
   // ทำ ถ้า year ไม่มีค่า และ month มีค่า
}else{ // ทำ ถ้า year ไม่มีค่า และ month ไม่มีค่า }