从MySQL中的表中选择最大ID

Question

I want the code to update the row "image" in my database to the name related to the picture the user chose to upload in their answer on my forum.

I want the query to take the question_id from the url and chose the highest answer id that is connected to that question_id. But the value keeps saying "NULL" it does not update.

  $mq = "
  UPDATE forum_answers 
     SET image = '".$_FILES['file']['name']."'
   WHERE question_id = '$id' 
     AND a_id = '(SELECT MAX(a_id) FROM forum_answers)'
       '
   ";
   $q = mysqli_query($conn,$mq);

My tables

Table: forum_questions
rows: id, topic, detail, name, datetime, reply, view
Sample value: id=45, topic=hej, detail=haha, name=admin, datetime=25/04/17 04:01:15, view=7, reply=0


Table: forum_answers
rows: question_id, a_id, a_name, a_answer, a_datetime, image

The image row here wont update.

Answer 1

Please check below code. WIsh it helps

 $file_name = $_FILES['file']['name'];
 $mq = "UPDATE forum_answers SET image= '$file_name' WHERE question_id='$id' 
 AND a_id=(SELECT MAX(a_id) FROM forum_answers)";

Answer 2

$mq = "UPDATE forum_answers SET image = '".$_FILES['file']['name']."' WHERE question_id=$id AND a_id='(SELECT MAX(a_id) FROM forum_answers)''";

请尝试这个question_id是整数类型