Question

Two simple scenarios:

$group_sql  = $db->prepare("UPDATE `groups` SET `group`=? WHERE `group_id`=?;");

foreach($_POST['groups'] as $id => $group)
{
    $group_sql->execute(array($group, $id));
}

And multi-update:

$query      = array();

foreach($_POST['groups'] as $id => $group)
{
    $query[]    = "WHEN {$id} THEN {$db->quote($group)}";
}

$query      = "
UPDATE `groups`
SET `group` = CASE `group_id`
    " . implode("\r\n", $query) . "
END
WHERE `group_id` IN (" . implode(',', array_keys($_POST['groups'])) . ");
";

Which will eventually generate:

UPDATE `groups`
SET `group` = CASE `group_id`
    WHEN 3054 THEN 'moteris savo vaikams'
    WHEN 3055 THEN 'bičių šeimoje'
END
WHERE `group_id` IN (3054,3055)

Which one is better to use?

UPDATE

Since I noticed that people are not familiar with such method, I’ve decided to expand the example to use multiple column/row update.

UPDATE `groups`
    SET `group` = CASE `group_id`
        WHEN 3054 THEN 'moteris savo vaikams'
        WHEN 3055 THEN 'bičių šeimoje'
    END,
    SET `description` = CASE `group_id`
        WHEN 3054 THEN 'foo'
        WHEN 3055 THEN 'bar'
    END
    WHERE `group_id` IN (3054,3055)

Answer 1

It depends entirely on your table and index structure. If group_id is index so that it can look it up directly, without scanning an index or a table it should yield similar performance.

If it isn’t indexed the last form should be able to handle with one index/table scan instead of two, which will get better performance.

Interesting idea to handle multiupdates that way, haven’t seen it before.