Undefined Index problem

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PHP

Hi,

Ive been having a problem trying to get around an Undefined Index notice.

Notice: Undefined index: cat_id in ...... on line 13

$sql = "SELECT
            cat_id,
            cat_name,
            cat_description
        FROM
            duk_categories
        WHERE
            cat_id = '" . mysql_real_escape_string($_GET['cat_id']) . "'";

This is the code there is highlighted by the notice. the bottom line is line 13. I read about isset but I'm still not sure how to do it and any help would be greatly appreciated.

I am passing the cat_id in the url (/category.php?2) from the previous page which is a discussion board where the user can select which category they want to view but the topics inside that category arent being shown.

Instead, this is the code that is being executed which tells the user the category doesnt exist (so there cant be any topics either):

if(mysql_num_rows($result) == 0)
    {
        echo 'This category does not exist.';
    }