db_merge() example

db_merge_example.php

<?php
// Track # of places sold for an event. The event is addressed by $entity_id and $date.
// $quantity is a positive number of places sold or negative, if places are cancelled.

  db_merge('places_sold')
    ->key(
      array(
        'entity_id' => $entity_id,
        'date' => $date->format(DATE_FORMAT_DATE),
      )
    )
    ->fields(array(
      'quantity' => max(0, $quantity),
    ))
    ->expression('quantity', "GREATEST(0, quantity + $quantity)")
    ->execute();