Updating display order of multiple MySQL rows in one or very few queries

Question

I have a table with say 20 rows each with a number for display order (1-20).

SELECT * FROM `mytable` ORDER BY `display_order` DESC;

From an adm

Answer 1

This is a great way of sorting items on a database. Exactly what i was looking for ;)

Here's an improved version of Jamon Holmgren's version. This function is totally dynamic:

function reOrderRows($tablename, $ordercol, $idsarray){

    $query = "UPDATE $tablename SET $ordercol = (CASE $ordercol ";
    foreach($idsarray as $prev => $new) {
      $query .= " WHEN $prev THEN $new\n";
    }
    $query .= " END) WHERE $ordercol IN (" . implode(",", array_keys($idsarray)) . ")";

    mysql_query($query);
}

This assumes that you have a column $ordercol on your table $tablename just for ordering purposes.

The $idsarray is an array in the format array("current_order_num" => "updated_order_num").

So if you just want to swap two lines in your table (imagine that you have for example a "move up" and "move down" icons on your webpage), you can use this function on top of the previous one:

function swapRows($tablename, $ordercol, $firstid, $secondid){
    $swaparray = array("$firstid" => "$secondid", "$secondid" => "$firstid");
    reOrderRows($tablename, $ordercol, $swaparray); 
}