Warning: mysql_num_rows() expects parameter 1 to be resource, boolean given [duplicate]

Warning: mysql_num_rows() expects parameter 1 to be resource, boolean
given [duplicate]

This question already has an answer here:
mysql_fetch_array() expects parameter 1 to be resource, boolean given in
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I don't know why it keeps telling me
mysql_num_rows() expects parameter 1 to be resource, boolean given in
C:\xampp\htdocs\schedule2\course.php on line 44
if(isset ($_POST['course']) && isset($_POST['coursedesc']))
{
$course = $_POST['course'];
$coursedesc = $_POST['coursedesc'];
$query = mysql_query("SELECT * FROM ".tblurser." WHERE CourseCode
='".$course."' and CourseDescription = '".$coursedesc."'");
if(mysql_num_rows($query) > 0)
{
echo"<script>alert('Data already exist.');</script>";
}else{
if(empty($course) or empty($coursedesc))
{
echo "<script>
alert(\"all fields are
required \");
</script>";
}else{
if(mysql_query("INSERT INTO tblcourse
VALUES('','$course','$coursedesc')"))
{
}ELSE{
echo"<script>
alert(\"Please try again\");
</script>";
}
}
}
}