Question

I have following PHP loop + SQL Update query.

for ($i=0;$i<count($_POST['id']);$i++) {

    if(!isset($_POST['live'][$i])){
        $_POST['live'][$i] = "0";
    } else { $_POST['live'][$i] = "1"; }

    $id = ($_POST['id'][$i]);
    $live = ($_POST['live'][$i]);
    $usr2 = $_SESSION['usr'];
    $updated = date("F j, Y, g:i a",time()+60*60);

    $sql = "UPDATE news SET live = '$live', usr2 = '$usr2', updated = '$updated' WHERE id = $id";
    $result = mysql_query($sql);
    //echo $sql."<br />";


}
if($result) {
    header("location: notes.php");
    exit();

}else {
    die("Query failed");
}

How does it work:

• I’m submitting big form will ALL OF THE table rows.
• receiving this in different file as an array
• if $_POST['live'] is ‘not set’ – set it to ‘0’, if ‘set’ set it to 1
• update array data within for loop

How to UPDATE only the rows which have been actually been changed?

Those which value from $_POST[‘live’] is actually different from this saved in DB, as the condition would be change of our $live row.

Answer 1

I guess you’re concerned about the updated field and that this value only changes when something has been altered. (If that’s not the case forget about this answer.)
You can define an ON UPDATE CURRENT_TIMESTAMP clause for a timestamp field. Each time a record is updated without explicitly setting a value for this field mysql uses the current time as its new value…
…but only if the record is altered; if you “update” the fields with the same value as are already in that record nothing happens.

demo script:

<?php
$pdo = new PDO('mysql:host=localhost;dbname=test;charset=utf8', 'localonly', 'localonly');
$pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
$pdo->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);

setup($pdo);
$stmt = $pdo->prepare('UPDATE soNews SET somevalue=:v WHERE id=:id');
show('original', $pdo);

$stmt->execute( array(':id'=>1, ':v'=>'z') );
show('update with new=old & id=1', $pdo);

$stmt->execute( array(':id'=>2, ':v'=>'y') ); // new value=old value
show('update with new!=old & id=2', $pdo);



function setup($pdo) {
    $pdo->exec('
        CREATE TEMPORARY TABLE soNews ( 
            id int auto_increment,
            somevalue varchar(32),
            updated TIMESTAMP DEFAULT 0 ON UPDATE CURRENT_TIMESTAMP,
            primary key(id)
        )
    ');

    $pdo->exec("INSERT INTO soNews (id,somevalue,updated) VALUES (1,'x', Now()-interval 47 hour),(2,'y', Now()-interval 47 hour)");
}

function show($label, $pdo) {
    echo "------ $label --------\n";
    foreach( $pdo->query('SELECT * FROM soNews', PDO::FETCH_ASSOC) as $row ) {
        echo join(', ', $row), "\n";
    }
}

prints

------ original --------
1, x, 2011-08-16 14:09:53
2, y, 2011-08-16 14:09:53
------ update with new=old & id=1 --------
1, z, 2011-08-18 13:09:53
2, y, 2011-08-16 14:09:53
------ update with new!=old & id=2 --------
1, z, 2011-08-18 13:09:53
2, y, 2011-08-16 14:09:53

As you can see as a result of the first update query the timestamp field has been updated while the second query setting new=old didn’t affect the updated field.