Duplicate Rows

This is how I check for duplicate rows:

$query="SELECT * FROM tablename WHERE number=$id AND yr=$yr";
$result=mysql_query($query,$db);
$num = mysql_affected_rows();

if($num>0)
{
echo "Error: Row already exists.";
}

What is the correc way to do this?

mysql_affected_rows does not work with SELECT statements. Instead, you should use mysql_num_rows. But, yes, other than that it looks good.