SELECT
co.id,
IFNULL( oe.follow_status, 'unFollow' ) AS followStatus,
co.due_time 
FROM
car_order co
LEFT JOIN order_expire oe ON oe.order_id = co.id 
WHERE
co.type = '贷款' 
AND co.STATUS = '已放款' 
AND co.approve_status != '结清成功' 
AND co.due_time 
You should sort by the IFNULL alias followStatus instead. 

Assuming due_time is a timestamp column, you should be able to compare it directly to a date literal:

SELECT co.id,
       IFNULL(oe.follow_status, 'unFollow') AS followStatus,
       co.due_time 
FROM car_order co
LEFT JOIN order_expire oe ON oe.order_id = co.id 
WHERE co.type = '贷款' AND 
      co.STATUS = '已放款' AND
      co.approve_status != '结清成功' AND
      co.due_time 
DATE_FORMAT() The function is used to convert date/timestamp to string, but it is not needed here as date/timestamp columns can be compared directly.