Warning:mysql_num_rows(): supplied argument is not a valid MySQL result resource นั้งแก้ไปแก้มาไม่ได้สักที ช่วยดูหน่อยครับว่าผิดยังไง

<?php
include "include/connectionDb.php";

$strID=$_POST["bID"];

$sql = "SELECT tbl_blog.*, tbl_user.*, DATE_FORMAT(tbl_blog.date_post, '%d-%m-%Y  %H:%i:%s') AS datepost FROM tbl_blog, tbl_user WHERE tbl_blog.user_id=tbl_user.user_id";

if($strID) {
	$sql .= " AND tbl_blog.user_id = '$strID'";
}
$sql .= " ORDER BY tbl_blog.blog_id DESC;";

$result = mysql_query($sql);

if(mysql_num_rows($result) == 0) {
	echo "ไม่พบบล็อกนี้";
}
else {
		echo "<strong><p align=\"center\">หัวข้อบล็อกของ <font color=#FF9900>$row[blog_name]</font></p></strong><br />";
			
	while($data = mysql_fetch_array($result)) {
		$blogname = $data['blog_name'];
		$blogid = $data['blog_id'];
		$title = $data['title'];
		$detail = $data['detail'];
		$userid = $data['user_id'];
		$date = $data['datepost'];
		
		$resultAns = mysql_query("SELECT COUNT(*) AS numans  FROM tbl_comment WHERE ref_blog_id='$blogid'"); 
		$row = mysql_fetch_array($resultAns);
		$numans = $row['numans'];
		
		echo "<a href=blog.php?bid=$blogid&userid=$userid target=_blank>$title</a><br>" . substr_replace($detail, "...", 100) . "<div style=\"font-size: 8pt; color: gray;\">โดย: <b>$blogname</b>  - ($numans ความคิดเห็น) - [$date]</div><br />";
	}
}	

mysql_close($objConnect);
?>