In MySQL (and MySQL only AFAIK), INSERT has a clause called ON DUPLICATE KEY UDPATE. When ON DUPLICATE KEY UPDATE is used with INSERT, the insert will update the record if a value for a unique or primary key already exists, or else create a record if the value does not exist. So now when a form can either create a new something or edit an existing something, you can use one query to do it, and not have to query to see if the something exists already.

So instead of this:

$num = $db->getone("select count(*) from events where event_id = 15");
if ($num > 0)
   $sql = "update events set name = 'New name' where event_id = 15";
   $sql = "insert into events (event_id, name) values (null, 'New name')";

you can do something like this:

$id = (empty($_POST['id'])) ? null : $_POST['id'];
$sql = "insert into events (event_id, name) values ($id, 'New name') on duplicate key update name = 'New name'";

Neat, eh?

In case you’re wondering what the difference is between ON DUPLICATE KEY UPDATE and a REPLACE query, a REPLACE fires a DELETE followed by an INSERT query, as opposed to a real UPDATE.