categories display problem show all categories

This Time Error Change

my table fields is
categories table and ( id pkey ) ( name var )

and articles table ( id pkey ) ( title var ) ( category_id int )

Code:

A PHP Error was encountered

Severity: Notice

Message: Undefined variable: category_name

Filename: models/article_model.php

Line Number: 83

PHP Code:

A Database Error Occurred

Error Number: 1054

Unknown column 'DESC' in 'order clause'

SELECT `a`.*, `c`.`name` as category_name FROM (`articles` as a) LEFT JOIN `categories` AS c ON `a`.`category_id` = `c`.`id` ORDER BY `DESC` DESC

Filename: /var/www/html/post/models/article_model.php

Line Number: 92 


This Time Error Change

Please Solve My Problem

my table fields is
categories table and ( id pkey ) ( name var )

and articles table ( id pkey ) ( title var ) ( category_id int )