ORDER BY column_name help（通过HTML表视图中的链接）（PHP MySQL

My output for my table in HTML has several columns such as userid, name, age, dob.

The table heading is simply the title of the column name, I want this to be a link, and when clicked, the selected column is sorted in order, ASC, and then DESC (on next click). I thought this was pretty straight forward but I'm having some difficulty.

So far, I have produced this, and no output is taken, apart from the URL works by displaying 'users.php?orderby=userid'

<?php
          if(isset($_GET['orderby'])){ 
$orderby = $_GET['orderby']; 
$query_sv = "SELECT * FROM users BY ".mysql_real_escape_string($orderby)." ASC"; 
} 
//default query 
else{ 
$query_sv = "SELECT * FROM users BY user_id DESC"; 
} 
?>


              <tr>
                <th><a href="<?php echo $_SERVER['php_SELF']."?orderby=userid";?>">User ID</a></th>

Hoefully if I get this working, I can sort the users by D.O.B. next also using the same principles. Does anyone have any ideas?

$orders=array("name","price","qty");
$key=array_search($_GET['orderby'],$orders));
$orderby=$orders[$key];
$query="SELECT * FROM `table` ORDER BY $orderby";

It should be SELECT * FROM users ORDER BY ... instead of SELECT * FROM users BY .... Also you might want to check that the passed parameter is a valid column name, otherwise your query will fail.

To add a direction would be more complicated.

First you need to make a conditional parameter.

if (empty($_GET['asc'])) {
  $orderby.=" DESC"; 
  $dir="&asc=1";
} else {
  $orderby.=" ASC"; 
  $dir="";
}

Now put $orderby into query and $dir into link:

<a href="?orderby=userid<?php echo $dir?>">