MySQL order by problems

问题

I have the following codes..

echo "<form><center><input type=submit name=subs value='Submit'></center></form>";

$val=$_POST['resulta']; //this is from a textarea name='resulta'
if (isset($_POST['subs'])) //from submit name='subs'
{
    $aa=mysql_query("select max(reservno) as 'maxr' from reservation") or die(mysql_error()); //select maximum reservno
    $bb=mysql_fetch_array($aa);
    $cc=$bb['maxr'];
    $lines = explode("\n", $val);
    foreach ($lines as $line) {
        mysql_query("insert into location_list (reservno, location) values ('$cc', '$line')")
          or die(mysql_error()); //insert value of textarea then save it separately in location_list if \n is found
    }

If I input the following data on the textarea (assume that I have maximum reservno '00014' from reservation table),

Davao - Cebu
Cebu - Davao

then submit it, I'll have these data in my location_list table:

loc_id || reservno || location
00001  || 00014    || Davao - Cebu
00002  || 00014    || Cebu - Davao

Then this code:

$gg=mysql_query("SELECT  GROUP_CONCAT(IF((@var_ctr := @var_ctr + 1) = @cnt, 
                            location, 
                            SUBSTRING_INDEX(location,' - ', 1)
                           ) 
                           ORDER BY loc_id ASC
                           SEPARATOR ' - ') AS locations

    FROM location_list, 
         (SELECT @cnt := COUNT(1), @var_ctr := 0 
          FROM location_list 
          WHERE reservno='$cc'
         ) dummy
    WHERE reservno='$cc'") or die(mysql_error()); //QUERY IN QUESTION
    $hh=mysql_fetch_array($gg);
    $ii=$hh['locations'];
    mysql_query("update reservation set itinerary = '$ii' where reservno = '$cc'")
      or die(mysql_error());

is supposed to update reservation table with 'Davao - Cebu - Davao' but it's returning this instead, 'Davao - Cebu - Cebu'. I was previously helped by this forum to have this code working but now I'm facing another difficulty. Just can't get it to work. Please help me. Thanks in advance!

回答1:

I got it working (without ORDER BY loc_id ASC) as long as I set phpMyAdmin operations loc_id ascending. But whenever I delete all data, it goes back as loc_id descending so I have to reset it. It doesn't entirely solve the problem but I guess this is as far as I can go. :)) I just have to make sure that the table column loc_id is always in ascending order. Thank you everyone for your help! I really appreciate it! But if you have any better answer, like how to set the table column always in ascending order or better query, etc, feel free to post it here. May God bless you all!

回答2:

The database server is allowed to rewrite your query to optimize its execution. This might affect the order of the individual parts, in particular the order in which the various assignments are executed. I assume that some such reodering causes the result of the query to become undefined, in such a way that it works on sqlfiddle but not on your actual production system.

I can't put my finger on the exact location where things go wrong, but I believe that the core of the problem is the fact that SQL is intended to work on relations, but you try to abuse it for sequential programming. I suggest you retrieve the data from the database using portable SQL without any variable hackery, and then use PHP to perform any post-processing you might need. PHP is much better suited to express the ideas you're formulating, and no optimization or reordering of statements will get in your way there. And as your query currently only results in a single value, fetching multiple rows and combining them into a single value in the PHP code shouldn't increase complexety too much.

---

Edit:

While discussing another answer using a similar technique (by Omesh as well, just as the answer your code is based upon), I found this in the MySQL manual:

As a general rule, you should never assign a value to a user variable and read the value within the same statement. You might get the results you expect, but this is not guaranteed. The order of evaluation for expressions involving user variables is undefined and may change based on the elements contained within a given statement; in addition, this order is not guaranteed to be the same between releases of the MySQL Server.

So there are no guarantees about the order these variable assignments are evaluated, therefore no guarantees that the query does what you expect. It might work, but it might fail suddenly and unexpectedly. Therefore I strongly suggest you avoid this approach unless you have some relaibale mechanism to check the validity of the results, or really don't care about whether they are valid.

来源：https://stackoverflow.com/questions/12243779/mysql-order-by-problems